1372. Longest ZigZag Path in a Binary Tree(DFS/DP)

ZustinLau

于 2022-06-06 05:02:36 发布

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分类专栏：動態規劃 Leetcode 文章标签：深度优先算法动态规划

本文链接：https://blog.youkuaiyun.com/weixin_58286631/article/details/125139833

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该博客主要探讨了一种解决二叉树中最长之字路径问题的方法。通过两次深度优先搜索（DFS），一次从左开始，一次从右开始，来确定最长路径。在遍历过程中，根据当前节点的左右子节点来调整路径长度，并用全局变量记录最大路径长度。难点在于初始化状态的设定和正确判断路径方向。

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simple dfs problem, if we change the direction, we add 1 to path length, otherwise reset the path length.

of course, we need to know the last node is left or right to judge we have adjusted the direction or not.

The hardest thing of this question is to think about the initial state.

we will do twice dfs, one assume we start from left, one assum we start from right.

class Solution {
public:
    //left = 1, right = 0;
    int ans = 0;
    void dfs(TreeNode *root, int last, int len){
        ans = max(len, ans);
        if(!root)return;
        if(root->left){
            if(!last)dfs(root->left, 1, len+1);
            else dfs(root->left, 1, 1);
        }
        if(root->right){
            if(last)dfs(root->right, 0, len+1);
            else dfs(root->right, 0, 1);
        }
    }
    int longestZigZag(TreeNode* root) {
        dfs(root, 1, 0);
        dfs(root, 0, 0);
        return ans;
    }
};