LeetCode.687 Longest Univalue Path （经典的求相同节点最长路径）

题目：

Given a binary tree, find the length of the longest path where each node in the path has the same value. This path may or may not pass through the root.

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

Example 1:

Input:

5
/ \
4   5
/ \   \
1   1   5

Output:

2

Example 2:

Input:

1
/ \
4   5
/ \   \
4   4   5

Output:

2

Note:The given binary tree has not more than 10000 nodes. The height of the tree is not more than 1000.

分析：

/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
class Solution {
private int max=0;
public int longestUnivaluePath(TreeNode root) {
//给定二叉树，找出一条所有节点均为相同值的最长路径。
//思路：递归查找是否同根节点相同。定义一个全局变量
//注意：递归返回最长的一边，即单边最大值
if(root==null) return 0;
backtrace(root);
return max;
}

//递归
public int backtrace(TreeNode root){
int left=root.left!=null?backtrace(root.left):0;
int right=root.right!=null?backtrace(root.right):0;
//递归判断是否根节点等于左右
int resLeft=root.left!=null&&root.val==root.left.val?left+1:0;
int resRight=root.right!=null&&root.val==root.right.val?right+1:0;

max=Math.max(max,resLeft+resRight);
//返回最长的一边
return Math.max(resLeft,resRight);
}
}