814 Binary Tree Pruning¶

We are given the head node root of a binary tree, where additionally every node’s value is either a 0 or a 1.

Return the same tree where every subtree (of the given tree) not containing a 1 has been removed.

(Recall that the subtree of a node X is X, plus every node that is a descendant of X.)

Example 1:
Input: [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: [1,0,1,0,0,0,1]
Output: [1,null,1,null,1]

Example 3:
Input: [1,1,0,1,1,0,1,0]
Output: [1,1,0,1,1,null,1]

Solution: Recursion¶

Time complexity: O(n) Space complexity: O(h)

// Running time: 2 ms
class Solution {
  public TreeNode pruneTree(TreeNode root) {
    if (root == null) return root;
    root.left = pruneTree(root.left);
    root.right = pruneTree(root.right);
    return (root.val == 1 || root.left != null || root.right != null) ? root : null;
  }
}