250. 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.

Solution：递归 post-order 分治 upwards上传

思路：
Divide: left sub and right sub
Conquer: ifSame(boolean left, boolean right), int count, and then uploads
Time Complexity:T(N) = 2T(N / 2) + O(1) => O(N)
Space Complexity: O(N)

Solution Code：

public class Solution {
    public int countUnivalSubtrees(TreeNode root) {
        int[] count = new int[1];
        helper(root, count);
        return count[0];
    }
    
    private boolean helper(TreeNode node, int[] count) {
        if (node == null) return true;

        // divide
        boolean left = helper(node.left, count);
        boolean right = helper(node.right, count);
        
        // conquer (ifSame(boolean left, boolean right), int count), 
        // and uploads ifSame & count 
        if (left && right) {
            if (node.left != null && node.val != node.left.val) {
                return false;
            }
            if (node.right != null && node.val != node.right.val) {
                return false;
            }
            count[0]++;
            return true;
        }
        return false;
    }
}