0814. Binary Tree Pruning

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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.

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]

Constraints:

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

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解法1

用递归.

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public TreeNode pruneTree(TreeNode root) {
        if(root == null)
            return null;
        
        root.left = pruneTree(root.left);
        root.right = pruneTree(root.right);
        
        if(root.left == null && root.right == null && root.val == 0)
            return null;
        
        return root;
    }
}