814. Binary Tree Pruning

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814. Binary Tree Pruning

题目描述

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:

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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:

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

Example 3:

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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.

题目大意

给定以二叉树，每个节点值为 0 或者 1。
删除每个不包含 1 的子树。

解题思路

遍历二叉树，如果根节点值为 1 或者 左子树不为空 或者 右子树不为空，返回 根节点，否则返回nil。

代码

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class Solution {
    func pruneTree(_ root: TreeNode?) -> TreeNode? {
        guard let tree = root else { return nil }
        tree.left = pruneTree(tree.left)
        tree.right = pruneTree(tree.right)
        return (tree.val == 1 || root?.left != nil || root?.right != nil) ? tree : nil
    }
}
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