1372

1372. Longest ZigZag Path in a Binary Tree (Medium)

Given a binary tree root, a ZigZag path for a binary tree is defined as follow:

• Choose any node in the binary tree and a direction (right or left).
• If the current direction is right then move to the right child of the current node otherwise move to the left child.
• Change the direction from right to left or right to left.
• Repeat the second and third step until you can't move in the tree.

Zigzag length is defined as the number of nodes visited - 1. (A single node has a length of 0).

Return the longest ZigZag path contained in that tree.

 

Example 1:

Input: root = [1,null,1,1,1,null,null,1,1,null,1,null,null,null,1,null,1]
Output: 3
Explanation: Longest ZigZag path in blue nodes (right -> left -> right).

Example 2:

Input: root = [1,1,1,null,1,null,null,1,1,null,1]
Output: 4
Explanation: Longest ZigZag path in blue nodes (left -> right -> left -> right).

Example 3:

Input: root = [1]
Output: 0

 

Constraints:

• Each tree has at most 50000 nodes..
• Each node's value is between [1, 100].

Related Topics:
Dynamic Programming, Tree

Solution 1.

// OJ: https://leetcode.com/problems/longest-zigzag-path-in-a-binary-tree
// Author: github.com/lzl124631x
// Time: O(N)
// Space: O(H)
class Solution {
    int A = 0;
    pair<int, int> postorder(TreeNode* root)  {
        if (!root || (!root->left && !root->right)) return { 0, 0 };
        pair<int, int> ans;
        if (root->left) {
            auto left = postorder(root->left);
            ans.first = 1 + left.second;
        }
        if (root->right) {
            auto right = postorder(root->right);
            ans.second = 1 + right.first;
        }
        A = max({ A, ans.first, ans.second });
        return ans;
    }
public:
    int longestZigZag(TreeNode* root) {
        postorder(root);
        return A;
    }
};