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;
}
}