814.cpp

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution { // Runtime: 4 ms, faster than 91.95% of C++ online submissions
                 // for Binary Tree Pruning.
public:
  TreeNode *pruneTree(TreeNode *root) {
    function<bool(TreeNode *)> postorder = [&postorder](TreeNode *node) {
      if (!node)
        return true;
      auto removeThis = node->val == 0;
      auto removeL = postorder(node->left);
      if (removeL)
        node->left = nullptr;
      auto removeR = postorder(node->right);
      if (removeR)
        node->right = nullptr;
      removeThis = removeThis and removeL and removeR;
      return removeThis;
    };
    return postorder(root) ? nullptr : root;
  }
};