leetcode-814-Binary-Tree-Pruning

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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.)

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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.

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Example 2:
Input: [1,0,1,0,0,0,1]
Output: [1,null,1,null,1]

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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.

分析

深度优先遍历，优先移除叶子节点，就可以用递归解决。

解决方案1(Java)

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/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
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;
    }
}

题目来源