Diameter of Binary Tree

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 thelongestpath between any two nodes in a tree. This path may or may not pass through the root.

Example:
Given a binary tree

          1
         / \
        2   3
       / \     
      4   5

Return3, 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.

题意：已知直径就是两点之间的最远距离，求二叉树的直径

图解

分析

1. length of longest path which pass it = MaxDepth of its left subtree + MaxDepth of its right subtree

代码

class Solution {
    private int res = 0;

    public int diameterOfBinaryTree(TreeNode root) {
        helper (root);
        return res;  
    }

    public int helper(TreeNode root){
        if(root == null) return 0;
        int left = helper(root.left);
        int right = helper(root.right);
        res = Math.max(res, left+right);
        return Math.max(left, right) +1;
    }
}