原題鏈接https://leetcode.com/problems/longest-univalue-path

題目描述

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

返回給定二叉樹的相同節點值的最長路徑，不一定要從根節點開始，且長度以節點之間的邊數表示。
對于某個節點

• 遞歸計算左右子樹的最大長度
• 更新當前最大長度
• 若左右子樹的根節點的值等于當前節點值的話，取大者
  All in all,很友好的題，但是我寫題解&&做題的時間超時了……

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;

    }

}

總結

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