Formatted question description: https://leetcode.ca/all/1372.html

1372. Longest ZigZag Path in a Binary Tree (Medium)

Given a binary tree root, 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 then move to the right child of the current node otherwise move to the left child.
• Change the direction from right to left or right to left.
• Repeat the second and third step 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:

• Each tree has at most 50000 nodes..
• Each node's value is between [1, 100].

Related Topics:
Dynamic Programming, Tree

Solution 1.

// OJ: https://leetcode.com/problems/longest-zigzag-path-in-a-binary-tree

// Time: O(N)
// Space: O(H)
class Solution {
    int A = 0;
    pair<int, int> postorder(TreeNode* root)  {
        if (!root || (!root->left && !root->right)) return { 0, 0 };
        pair<int, int> ans;
        if (root->left) {
            auto left = postorder(root->left);
            ans.first = 1 + left.second;
        }
        if (root->right) {
            auto right = postorder(root->right);
            ans.second = 1 + right.first;
        }
        A = max({ A, ans.first, ans.second });
        return ans;
    }
public:
    int longestZigZag(TreeNode* root) {
        postorder(root);
        return A;
    }
};

Java

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public int longestZigZag(TreeNode root) {
        Map<TreeNode, int[]> map = new HashMap<TreeNode, int[]>();
        map.put(root, new int[]{0, 0});
        int maxLength = 0;
        Queue<TreeNode> queue = new LinkedList<TreeNode>();
        queue.offer(root);
        while (!queue.isEmpty()) {
            TreeNode node = queue.poll();
            int[] lengths = map.get(node);
            TreeNode left = node.left, right = node.right;
            if (left != null) {
                int[] leftLengths = new int[2];
                leftLengths[0] = lengths[1] + 1;
                maxLength = Math.max(maxLength, leftLengths[0]);
                map.put(left, leftLengths);
                queue.offer(left);
            }
            if (right != null) {
                int[] rightLengths = new int[2];
                rightLengths[1] = lengths[0] + 1;
                maxLength = Math.max(maxLength, rightLengths[1]);
                map.put(right, rightLengths);
                queue.offer(right);
            }
        }
        return maxLength;
    }
}

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