814. Binary Tree Pruning

1. Description

Given the root of a binary tree, return the same tree where every subtree (of the given tree) not containing a 1 has been removed.
A subtree of a node node is node plus every node that is a descendant of node.

2. Example

Example 1:

Input: root = [1,null,0,0,1]
Output: [1,null,0,null,1]
Explanation:
Only the red nodes satisfy the property “every subtree not containing a 1”.
The diagram on the right represents the answer.

Example 2:

Input: root = [1,0,1,0,0,0,1]
Output: [1,null,1,null,1]

Example 3:

Input: root = [1,1,0,1,1,0,1,0]
Output: [1,1,0,1,1,null,1]

3. Constraints

• The number of nodes in the tree is in the range [1, 200].
• Node.val is either 0 or 1.

4. Solutions

Recursion && Post-Order Iteration

n is the number of nodes in root
Time complexity: O(n)
Space complexity: O(n)

class Solution {
public:
    TreeNode *pruneTree(TreeNode *root) {
        if(root == nullptr) {
            return nullptr;
        }

        root->left = pruneTree(root->left);
        root->right = pruneTree(root->right);

        if(root->left == nullptr && root->right ==nullptr && root->val == 0) {
            return nullptr;
        }

        return root;
    }
};