1372. Longest ZigZag Path in a Binary Tree

You are given the root of a binary tree.

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, move to the right child of the current node; otherwise, move to the left child.
• Change the direction from right to left or from left to right.
• Repeat the second and third steps 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.

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
    int res = 0;
    int helper(TreeNode* node, bool amIRight) {
        if(!node) return 0;
        if(amIRight) {
            res = max(res, 1 + helper(node->right, true));
            return 1 + helper(node->left, false);
        } else {
            res = max(res, 1 + helper(node->left, false));
            return 1 + helper(node->right, true);
        }
    }
public:
    int longestZigZag(TreeNode* root) {
        res = max(res, 1 + helper(root->left, false));
        res = max(res, 1 + helper(root->right, true));
        return res - 1;
    }
};