814

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    TreeNode* pruneTree(TreeNode* root) {
        return dfs(root) ? root : NULL;
    }

    bool dfs(TreeNode* root){
        if(!root) return false;

        bool left = dfs(root->left);
        bool right = dfs(root->right);

        if(!left) root->left = NULL;
        if(!right) root->right = NULL;

        return (root->val == 1 || left || right);
    }
};