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);
        }
    }
}