原題鏈接在這里：https://leetcode.com/problems/count-univalue-subtrees/

題目：

Given a binary tree, count the number of uni-value subtrees.

A Uni-value subtree means all nodes of the subtree have the same value.

For example:
Given binary tree,

5/ \1 5/ \ \5 5 5?

return?4.

題解：

bottom-up recursion. dfs返回當前root下是不是univalue tree.

Time Complexity: O(n).

Space: O(logn). height of tree.

AC Java:

1 /** 2 * Definition for a binary tree node. 3 * public class TreeNode { 4 * int val; 5 * TreeNode left; 6 * TreeNode right; 7 * TreeNode(int x) { val = x; } 8 * } 9 */ 10 class Solution { 11 public int countUnivalSubtrees(TreeNode root) { 12 int [] res = {0}; 13 dfs(root, res); 14 return res[0]; 15 } 16 17 private boolean dfs(TreeNode root, int [] res){ 18 if(root == null){ 19 return true; 20 } 21 22 boolean left = dfs(root.left, res); 23 boolean right = dfs(root.right, res); 24 if(left && right){ 25 if(root.left!=null && root.left.val!=root.val){ 26 return false; 27 } 28 29 if(root.right!=null && root.right.val!=root.val){ 30 return false; 31 } 32 33 res[0]++; 34 return true; 35 } 36 37 return false; 38 } 39 }

類似Longest Univalue Path.

轉載于:https://www.cnblogs.com/Dylan-Java-NYC/p/5187437.html

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