1372.cpp

/**
 * 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 ans;
public:
    int _longestZigZag(TreeNode *root, bool dir) {
        if (!root) {
            return -1;
        }

        int left = _longestZigZag(root->left, false);
        int right = _longestZigZag(root->right, true);

        ans = max(ans, 1 + max(left, right));
        return (dir ? left : right) + 1;
    }

    int longestZigZag(TreeNode* root) {
        ans = 0;
        _longestZigZag(root, false);
        return ans;
    }
};