​LeetCode刷题实战250：统计同值子树

Input: root = [5,1,5,5,5,null,5]

              5
             /  \
            1   5
           /  \    \
          5   5   5

Output: 4

class Solution {
public:
    int countUnivalSubtrees(TreeNode* root) {
        int sum=0;
        helper(root,sum);
        return sum;
    }
    bool helper(TreeNode* node,int& sum){
        if(node==0){
            return true;
        }
        bool le=helper(node->left,sum),ri=helper(node->right,sum);
        if(le and ri){
            if(node->left and node->left->val!=node->val){
                return false;
            }
            if(node->right and node->right->val!=node->val){
                return false;
            }
            ++sum;
            return true;
        }
        return false;
    }
};