814. 二叉树剪枝

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/**
 * 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) {
        return dfs(root);
    }

    public TreeNode dfs(TreeNode root){
        if(root == null){
            return null;
        }

        if(root.left != null){
            root.left = dfs(root.left);
        }

        if(root.right != null){
            root.right = dfs(root.right);
        }

        if(root.val == 0 && root.left == null && root.right == null){
            root = null;
        }
        return root;
    }
}