原题链接https://leetcode.com/problems/longest-univalue-path

题目描述

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.

思路分析

返回给定二叉树的相同节点值的最长路径，不一定要从根节点开始，且长度以节点之间的边数表示。
对于某个节点

• 递归计算左右子树的最大长度
• 更新当前最大长度
• 若左右子树的根节点的值等于当前节点值的话，取大者
  All in all,很友好的题，但是我写题解&&做题的时间超时了……

AC解

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {    
    int res = 0;
    public int longestUnivaluePath(TreeNode root) {
        if(root == null)
        {
            return 0;
        }
        
        dfs(root,0);
        return res;
    }
    
    public int dfs(TreeNode root,int curVal)
    {
        if(root == null)
        {
            return 0;
        }
        
        int left = (root.left != null) ? dfs(root.left,root.val) : 0;
        int right = (root.right != null) ? dfs(root.right,root.val) : 0;
        
        res = Math.max(res,left + right);        
       
        return (curVal == root.val) ? (Math.max(left,right)+1) : 0;
    }
}