250. Count Univalue Subtrees

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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,

1
2
3
4
5
    5
   / \
  1   5
 / \   \
5   5   5

return 4.

解法1：

用Divide & Conquer
C++

1

Java

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/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    int res = 0;
    public int countUnivalSubtrees(TreeNode root) {
        helper(root);
        return res;
    }
    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;
            }
            ++res;
            return true;            
        } else {
            return false;
        }
    }
}