1372. Longest ZigZag Path in a Binary Tree

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# 題目敘述

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

# 解題思路

# Solution

class Solution {

    int ans = 0;

    public int longestZigZag(TreeNode root) {

        dfs(root, false, 0);

        dfs(root, true, 0);

        return ans;

    }

    public void dfs(TreeNode node, boolean goLeft, int steps) {

        if (node == null) {

            return ;

        }

        ans = Math.max(ans, steps);

        if (goLeft) {

            dfs(node.left, false, steps + 1);

            dfs(node.right, true, 1);

        } else {

            dfs(node.left, false, 1);

            dfs(node.right, true, steps + 1);

        }

    }

}