Today I have done leetcode's April LeetCoding Challenge with cpp.

April LeetCoding Challenge 19

Description

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.

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:

• The number of nodes in the tree is in the range [1, 5 * 104].
• 1 <= Node.val <= 100

Solution

class Solution {
  int answer = 0;
  int solve(TreeNode *root, bool left) {
    if(!root) return 0;
    int r = solve(root->right, false);
    int l = solve(root->left, true);
    answer = max(answer, left + r);
    answer = max(answer, !left + l);
    if(left) return 1 + r;
    else return 1 + l;
  }
public:
  int longestZigZag(TreeNode* root) {
    solve(root->left, true);
    solve(root->right, false);
    return answer;
  }
};

// Accepted
// 58/58 cases passed (163 ms)
// Your runtime beats 84.67 % of cpp submissions
// Your memory usage beats 42.16 % of cpp submissions (100.4 MB)