Leetcode•Oct 22, 2026

Longest ZigZag Path in a Binary Tree

Hazrat Ali

Leetcode

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]
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

Solution
var longestZigZag = function(root) {
    let maxLength = 0;

    function dfs(node, left, right) {
        if (!node) return;

        maxLength = Math.max(maxLength, left, right);

        dfs(node.left, right + 1, 0);

        dfs(node.right, 0, left + 1);
    }

    dfs(root, 0, 0);

    return maxLength;
};
 

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