From 5f7e37b79b6d8d15de520f5151fad9944c268261 Mon Sep 17 00:00:00 2001 From: deeaitch Date: Wed, 17 Jun 2026 18:01:14 -0400 Subject: [PATCH] Ref: 2 memory walkthrough was added --- .../exapmle/reverse-linked-list/README.md | 5 + .../reverse-linked-list/memory_walkthrough.md | 827 ++++++++++++++++++ 2 files changed, 832 insertions(+) create mode 100644 analysis/04-reverse-linked-list/exapmle/reverse-linked-list/memory_walkthrough.md diff --git a/analysis/04-reverse-linked-list/exapmle/reverse-linked-list/README.md b/analysis/04-reverse-linked-list/exapmle/reverse-linked-list/README.md index a76ec4a..eafe4cf 100644 --- a/analysis/04-reverse-linked-list/exapmle/reverse-linked-list/README.md +++ b/analysis/04-reverse-linked-list/exapmle/reverse-linked-list/README.md @@ -23,3 +23,8 @@ Original list: Reversed list: 5 -> 4 -> 3 -> 2 -> 1 -> null ``` + + +## Memory walkthrough + +see memory_walkthrough.md \ No newline at end of file diff --git a/analysis/04-reverse-linked-list/exapmle/reverse-linked-list/memory_walkthrough.md b/analysis/04-reverse-linked-list/exapmle/reverse-linked-list/memory_walkthrough.md new file mode 100644 index 0000000..deb2a50 --- /dev/null +++ b/analysis/04-reverse-linked-list/exapmle/reverse-linked-list/memory_walkthrough.md @@ -0,0 +1,827 @@ +# Reverse Linked List — Memory Walkthrough + +This walkthrough explains the classic three-pointer linked list reversal using a memory-oriented view. + +The goal is not only to show that the algorithm works, but also to show what happens to: + +- stack variables +- heap nodes +- `next` fields inside each node + +The example list contains three nodes: + +```text +1 -> 2 -> 3 -> null +``` + +For clarity, fake addresses are used: + +```text +Node 1: 0x1000 +Node 2: 0x2000 +Node 3: 0x3000 +``` + +These addresses are illustrative only. +A real program will use different addresses. + +--- + +## Algorithm + +```cpp +Node* reverseList(Node* head) { + Node* previous = nullptr; + Node* current = head; + + while(current != nullptr) { + Node* next = current->next; + + current->next = previous; + + previous = current; + current = next; + } + + return previous; +} +``` + +The three important pointers are: + +| Pointer | Meaning | +|---|---| +| `previous` | Head of the already reversed part | +| `current` | Node currently being processed | +| `next` | Saved pointer to the remaining original list | + +The most important rule is: + +> Save `next` before changing `current->next`. + +Otherwise the rest of the original list may become unreachable. + +--- + +## Step 0 — Initial State + +Before the loop starts: + +```cpp +Node* previous = nullptr; +Node* current = head; +``` + +### Stack + +```text ++----------+----------+ +| Variable | Value | ++----------+----------+ +| head | 0x1000 | +| previous | nullptr | +| current | 0x1000 | +| next | not set | ++----------+----------+ +``` + +### Heap + +```text +0x1000 ++-----------+-----------+ +| value = 1 | next=2000 | ++-----------+-----------+ + +0x2000 ++-----------+-----------+ +| value = 2 | next=3000 | ++-----------+-----------+ + +0x3000 ++-----------+-----------+ +| value = 3 | next=null | ++-----------+-----------+ +``` + +### Logical view + +```text +head/current + | + v +1 -> 2 -> 3 -> null + +previous -> null +``` + +At this point, nothing has been reversed yet. + +--- + +## Step 1 — Save `next` + +Inside the first loop iteration: + +```cpp +Node* next = current->next; +``` + +`current` points to Node 1. +`current->next` points to Node 2. + +So: + +```text +next = 0x2000 +``` + +### Stack + +```text ++----------+----------+ +| Variable | Value | ++----------+----------+ +| head | 0x1000 | +| previous | nullptr | +| current | 0x1000 | +| next | 0x2000 | ++----------+----------+ +``` + +### Heap + +```text +0x1000 ++-----------+-----------+ +| value = 1 | next=2000 | ++-----------+-----------+ + +0x2000 ++-----------+-----------+ +| value = 2 | next=3000 | ++-----------+-----------+ + +0x3000 ++-----------+-----------+ +| value = 3 | next=null | ++-----------+-----------+ +``` + +Nothing in the heap changed yet. + +The `next` stack variable only saves access to the rest of the list. + +Without this temporary pointer, Node 2 and Node 3 could be lost after the next operation. + +--- + +## Step 2 — Reverse `current->next` + +Now the algorithm changes the link: + +```cpp +current->next = previous; +``` + +`current` is Node 1. +`previous` is `nullptr`. + +So Node 1 no longer points to Node 2. +It now points to `nullptr`. + +### Stack + +```text ++----------+----------+ +| Variable | Value | ++----------+----------+ +| head | 0x1000 | +| previous | nullptr | +| current | 0x1000 | +| next | 0x2000 | ++----------+----------+ +``` + +### Heap + +```text +0x1000 ++-----------+-----------+ +| value = 1 | next=null | ++-----------+-----------+ + +0x2000 ++-----------+-----------+ +| value = 2 | next=3000 | ++-----------+-----------+ + +0x3000 ++-----------+-----------+ +| value = 3 | next=null | ++-----------+-----------+ +``` + +### Logical view + +```text +current + | + v +1 -> null + +next + | + v +2 -> 3 -> null +``` + +This is the key mutation. + +The original list is now split into two logical parts: + +```text +Reversed part: +1 -> null + +Remaining original part: +2 -> 3 -> null +``` + +--- + +## Step 3 — Move `previous` + +The algorithm advances the reversed part: + +```cpp +previous = current; +``` + +`previous` now points to Node 1. + +### Stack + +```text ++----------+----------+ +| Variable | Value | ++----------+----------+ +| head | 0x1000 | +| previous | 0x1000 | +| current | 0x1000 | +| next | 0x2000 | ++----------+----------+ +``` + +### Heap + +```text +0x1000 ++-----------+-----------+ +| value = 1 | next=null | ++-----------+-----------+ + +0x2000 ++-----------+-----------+ +| value = 2 | next=3000 | ++-----------+-----------+ + +0x3000 ++-----------+-----------+ +| value = 3 | next=null | ++-----------+-----------+ +``` + +### Logical view + +```text +previous/current + | + v + 1 -> null + +next + | + v +2 -> 3 -> null +``` + +The reversed part now officially starts at `previous`. + +--- + +## Step 4 — Move `current` + +The algorithm continues with the saved next node: + +```cpp +current = next; +``` + +`current` now points to Node 2. + +### Stack + +```text ++----------+----------+ +| Variable | Value | ++----------+----------+ +| head | 0x1000 | +| previous | 0x1000 | +| current | 0x2000 | +| next | 0x2000 | ++----------+----------+ +``` + +### Heap + +```text +0x1000 ++-----------+-----------+ +| value = 1 | next=null | ++-----------+-----------+ + +0x2000 ++-----------+-----------+ +| value = 2 | next=3000 | ++-----------+-----------+ + +0x3000 ++-----------+-----------+ +| value = 3 | next=null | ++-----------+-----------+ +``` + +### Logical view + +```text +previous + | + v + 1 -> null + +current + | + v + 2 -> 3 -> null +``` + +The first iteration is complete. + +--- + +## Step 5 — Second Iteration: Save `next` + +The loop repeats. + +```cpp +Node* next = current->next; +``` + +`current` points to Node 2. +Node 2 points to Node 3. + +So: + +```text +next = 0x3000 +``` + +### Stack + +```text ++----------+----------+ +| Variable | Value | ++----------+----------+ +| head | 0x1000 | +| previous | 0x1000 | +| current | 0x2000 | +| next | 0x3000 | ++----------+----------+ +``` + +### Heap + +```text +0x1000 ++-----------+-----------+ +| value = 1 | next=null | ++-----------+-----------+ + +0x2000 ++-----------+-----------+ +| value = 2 | next=3000 | ++-----------+-----------+ + +0x3000 ++-----------+-----------+ +| value = 3 | next=null | ++-----------+-----------+ +``` + +Again, the heap has not changed yet. + +--- + +## Step 6 — Second Iteration: Reverse Link + +```cpp +current->next = previous; +``` + +`current` is Node 2. +`previous` is Node 1. + +So Node 2 now points back to Node 1. + +### Stack + +```text ++----------+----------+ +| Variable | Value | ++----------+----------+ +| head | 0x1000 | +| previous | 0x1000 | +| current | 0x2000 | +| next | 0x3000 | ++----------+----------+ +``` + +### Heap + +```text +0x1000 ++-----------+-----------+ +| value = 1 | next=null | ++-----------+-----------+ + +0x2000 ++-----------+-----------+ +| value = 2 | next=1000 | ++-----------+-----------+ + +0x3000 ++-----------+-----------+ +| value = 3 | next=null | ++-----------+-----------+ +``` + +### Logical view + +```text +current + | + v +2 -> 1 -> null + +next + | + v +3 -> null +``` + +The reversed part will become: + +```text +2 -> 1 -> null +``` + +after `previous` moves to Node 2. + +--- + +## Step 7 — Second Iteration: Move Pointers + +```cpp +previous = current; +current = next; +``` + +### Stack + +```text ++----------+----------+ +| Variable | Value | ++----------+----------+ +| head | 0x1000 | +| previous | 0x2000 | +| current | 0x3000 | +| next | 0x3000 | ++----------+----------+ +``` + +### Heap + +```text +0x1000 ++-----------+-----------+ +| value = 1 | next=null | ++-----------+-----------+ + +0x2000 ++-----------+-----------+ +| value = 2 | next=1000 | ++-----------+-----------+ + +0x3000 ++-----------+-----------+ +| value = 3 | next=null | ++-----------+-----------+ +``` + +### Logical view + +```text +previous + | + v + 2 -> 1 -> null + +current + | + v + 3 -> null +``` + +Now two nodes are reversed. + +--- + +## Step 8 — Third Iteration: Save `next` + +```cpp +Node* next = current->next; +``` + +`current` is Node 3. +Node 3 points to `nullptr`. + +So: + +```text +next = nullptr +``` + +### Stack + +```text ++----------+----------+ +| Variable | Value | ++----------+----------+ +| head | 0x1000 | +| previous | 0x2000 | +| current | 0x3000 | +| next | nullptr | ++----------+----------+ +``` + +### Heap + +```text +0x1000 ++-----------+-----------+ +| value = 1 | next=null | ++-----------+-----------+ + +0x2000 ++-----------+-----------+ +| value = 2 | next=1000 | ++-----------+-----------+ + +0x3000 ++-----------+-----------+ +| value = 3 | next=null | ++-----------+-----------+ +``` + +--- + +## Step 9 — Third Iteration: Reverse Link + +```cpp +current->next = previous; +``` + +`current` is Node 3. +`previous` is Node 2. + +So Node 3 now points to Node 2. + +### Stack + +```text ++----------+----------+ +| Variable | Value | ++----------+----------+ +| head | 0x1000 | +| previous | 0x2000 | +| current | 0x3000 | +| next | nullptr | ++----------+----------+ +``` + +### Heap + +```text +0x1000 ++-----------+-----------+ +| value = 1 | next=null | ++-----------+-----------+ + +0x2000 ++-----------+-----------+ +| value = 2 | next=1000 | ++-----------+-----------+ + +0x3000 ++-----------+-----------+ +| value = 3 | next=2000 | ++-----------+-----------+ +``` + +### Logical view + +```text +current + | + v +3 -> 2 -> 1 -> null +``` + +The whole list is now reversed, but the loop still needs to update the stack pointers. + +--- + +## Step 10 — Third Iteration: Move Pointers + +```cpp +previous = current; +current = next; +``` + +Since `next` is `nullptr`, `current` becomes `nullptr`. + +### Stack + +```text ++----------+----------+ +| Variable | Value | ++----------+----------+ +| head | 0x1000 | +| previous | 0x3000 | +| current | nullptr | +| next | nullptr | ++----------+----------+ +``` + +### Heap + +```text +0x1000 ++-----------+-----------+ +| value = 1 | next=null | ++-----------+-----------+ + +0x2000 ++-----------+-----------+ +| value = 2 | next=1000 | ++-----------+-----------+ + +0x3000 ++-----------+-----------+ +| value = 3 | next=2000 | ++-----------+-----------+ +``` + +### Logical view + +```text +previous + | + v + 3 -> 2 -> 1 -> null + +current -> null +``` + +The loop condition fails: + +```cpp +while(current != nullptr) +``` + +because `current` is now `nullptr`. + +--- + +## Step 11 — Return New Head + +At the end: + +```cpp +return previous; +``` + +`previous` points to Node 3. + +Node 3 is the new head of the reversed list. + +### Final stack view + +```text ++----------+----------+ +| Variable | Value | ++----------+----------+ +| old head | 0x1000 | +| new head | 0x3000 | ++----------+----------+ +``` + +### Final heap view + +```text +0x3000 ++-----------+-----------+ +| value = 3 | next=2000 | ++-----------+-----------+ + +0x2000 ++-----------+-----------+ +| value = 2 | next=1000 | ++-----------+-----------+ + +0x1000 ++-----------+-----------+ +| value = 1 | next=null | ++-----------+-----------+ +``` + +### Final logical view + +```text +new head + | + v + 3 -> 2 -> 1 -> null +``` + +--- + +## Why the Temporary `next` Pointer Matters + +This line is not optional: + +```cpp +Node* next = current->next; +``` + +Without it, this operation: + +```cpp +current->next = previous; +``` + +would overwrite the only pointer to the remaining original list. + +For example, at the beginning: + +```text +1 -> 2 -> 3 -> null +``` + +If Node 1 is changed to: + +```text +1 -> null +``` + +before saving Node 2, then Node 2 and Node 3 are no longer reachable from any local variable. + +That is why the algorithm always follows this order: + +```cpp +Node* next = current->next; // preserve the remaining list +current->next = previous; // reverse the link +previous = current; // grow the reversed part +current = next; // continue with the remaining part +``` + +The order is the algorithm. + +--- + +## Summary + +During the algorithm: + +- `previous` points to the already reversed part. +- `current` points to the node being processed. +- `next` preserves access to the not-yet-processed part. +- Only one `next` field is modified per iteration. +- No nodes are copied. +- No new list is allocated. +- The original nodes are relinked in-place. + +The algorithm is small, but it is easy to get wrong because it mutates the structure while traversing it. + +That is why a memory-level walkthrough is often more useful than just showing the final code.