[LeetCode] 543. Diameter of Binary Tree

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

1. Description

Given a binary tree, you need to compute 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.

Example:
Given a binary tree

Return 3, which is the length of the path [4,2,1,3] or [5,2,1,3].

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

2. Solution

For a node, the diameter of this node is left subtree’s depth plus right subtree’s depth.
Therefore we can use dfs to count every node’s depth and find the longest diameter.
code

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    int ans = 0;
    public int diameterOfBinaryTree(TreeNode root) {
        dfs(root);
        return ans;
    }
    public int dfs(TreeNode t ){
        if(t == null){
            return 0;
        }
        int l = dfs(t.left);
        int r = dfs(t.right);
        ans = Math.max(ans, l +r);
        return Math.max(l, r) +1;
    }
}