814. Binary Tree Pruning

題目

https://leetcode.com/problems/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]

Note:

• The binary tree will have at most 100 nodes.
• The value of each node will only be 0 or 1.

解答

這道題的意思是，一個二叉樹，裏面的節點只有0和1，讓我們將只包含0的節點都移除掉。

class P814_Binary_Tree_Pruning {

    public TreeNode pruneTree(TreeNode root) {
        noOne(root);
        return root;
    }

    public boolean noOne(TreeNode root) {
        if (root == null) {
            return true;
        } else {
            boolean leftNoOne = noOne(root.left);
            boolean rightNoOne = noOne(root.right);

            if (leftNoOne) {
                root.left = null;
            }
            if (rightNoOne) {
                root.right = null;
            }
            if (root.val != 1 && leftNoOne && rightNoOne) {
                return true;
            } else {
                return false;
            }
        }
    }

}