[leetcode #543] Diameter of Binary Tree

Problem

Given the root of a binary tree, return the length of the diameter of the tree.

The diameter of a binary tree is the length of the longest path between any two nodes in a tree. This path may or may not pass through the root.

The length of a path between two nodes is represented by the number of edges between them.

Example 1:

Input: root = [1,2,3,4,5]
Output: 3
Explanation: 3 is the length of the path [4,2,1,3] or [5,2,1,3].

Example 2:

Input: root = [1,2]
Output: 1

Constraints:

・ The number of nodes in the tree is in the range [1, 10⁴].
・ -100 <= Node.val <= 100

Idea

아주 쉬운 문제다. tree 탐색하면서 depth를 측정하고 backtracking을 통해 두 자식 노드 사이의 길이를 측정한다. 전체 노드를 모두 탐색한 후 나오는 길이의 최대값이 binary tree의 diameter다.

Solution

class Solution {
    int res;

    public int diameterOfBinaryTree(TreeNode root) {
        res = 0;

        bfs(root);
   
        return res;
    }

    private int bfs(TreeNode node) {
        if (node == null)
            return -1;

        int left = 1 + bfs(node.left);
        int right = 1 + bfs(node.right);

        int depth = Math.max(left, right);
        res = Math.max(res, left+right);
        return depth;
    }
}

Reference

https://leetcode.com/problems/diameter-of-binary-tree/