[leetcode]543. Diameter of Binary Tree二叉树直径

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 

          1
         / \
        2   3
       / \     
      4   5    

 

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

 

题目

给定一棵二叉树，求任意两个节点的最长路径长度。

 

思路

长度的定义是边的个数，不是node的个数

跟 [leetcode]124. Binary Tree Maximum Path Sum二叉树最大路径和 思路一致。

 

代码

class Solution {
    public int diameterOfBinaryTree(TreeNode root) {
        /* 要么用个global variable放在class下，要么用长度为1的一维数组来存。
           这里因为求edge的数量，初始化为一维数组的default值0是可行的。
        */
        int[] diameter = new int[1]; 
        dfs(root, diameter);
        return diameter[0];        
    }

    private int dfs(TreeNode node, int[] diameter) {
        if(node == null){return 0;}
        
        int lh = dfs(node.left, diameter);
        int rh = dfs(node.right, diameter);
        
        diameter[0] = Math.max(diameter[0], lh + rh);
        return Math.max(lh, rh) + 1;
    }
}