LC250. Count Univalue Subtrees
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.
recursion + 全局变量
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
public class Solution {
int count = 0;
public int countUnivalSubtrees(TreeNode root) {
if(root == null) return 0;
helper(root);
return count;
}
private boolean helper(TreeNode root){
if(root == null) return true;
boolean left = helper(root.left);
boolean right = helper(root.right);
if(left && right){
if(root.left != null && root.left.val != root.val) return false;
if(root.right != null && root.right.val != root.val) return false;
count++;
return true;
}
return false;
}
}
O(n), O(height)