递归

这篇是我做 78. Subsets · 子集 时整理的回溯（backtracking）笔记。一开始我能背模板，但说不清 path.pop() 到底在干嘛、为什么子集不会重复。后来配合动画一步步走，才真的看懂。 This is my note on backtracking, written while working on LeetCode 78 Subsets. At first I could recite the template, but I couldn’t explain what path.pop() was actually doing, or why subsets never come out duplicated. Stepping through it with animations is what finally made it click. 一句话总结本质： 回溯就是在一棵决策树上做 DFS。每走一步都是三件事：做选择（append）→ 往下走（递归）→ 撤销选择（pop）。撤销是为了回到岔路口，让下一个选项拿到一条干净的 path。 Backtracking is DFS on a decision tree. Every step is three moves: choose (append) → explore (recurse) → un-choose (pop). Un-choosing walks you back to the fork, so the next option starts from a clean path. ...

这篇是我做 124. Binary Tree Maximum Path Sum · 二叉树中的最大路径和 时整理的笔记。卡住我的不是公式，而是递归到底按什么顺序走。 This is a note from working on LeetCode 124. What tripped me up wasn’t the formula. It was the order the recursion actually runs in. 一句话总结本质： 每进入一个节点，先一路往左钻到底；左边返回后，同一个节点才去右边；进了右子树，又重新「先左后右」。两边都返回后，才处理自己。完整规则：左 → 右 → 自己（后序遍历，post-order）。 Every time you enter a node, you dive all the way left first. Once the left side returns, that same node goes right. Inside the right subtree, “left first, then right” starts over. Only after both sides return does the node process itself. The full rule: left → right → self (post-order). ...