LeetCode75-1372-ZigZag

🧠 Summary of Thoughts

ZigZag = alternate walking left and right

1. Use Depth-First Search (DFS) to traverse the tree recursively.
2. Track the current path length separately for left and right directions.
3. At each node:

• Update the maximum ZigZag length.
• Recurse to the left child with the path continuing from the right direction.
• Recurse to the right child with the path continuing from the left direction.

4. Maintain a global or class-level variable to store the maximum path length found.

✅ Code（Java）

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    int max = 0;
    public int longestZigZag(TreeNode root) {
        dfs(root, 0, 0);
        return max;
    }
    private void dfs(TreeNode node, int leftLen, int rightLen) {
        if (node == null) {
            return;
        }
        max = Math.max(max, Math.max(leftLen, rightLen));
        dfs(node.left, rightLen + 1, 0);
        dfs(node.right, 0, leftLen + 1);
    }
}