[LeetCode]226. 翻轉二叉樹

題目

翻轉一棵二叉樹。

示例：

輸入：

     4
   /   \
  2     7
 / \   / \
1   3 6   9

輸出：

     4
   /   \
  7     2
 / \   / \
9   6 3   1

解題思路

詳細思路請參考 面試題27. 二叉樹的鏡像

代碼

解法一：遞歸法：

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

class Solution {
    public TreeNode invertTree(TreeNode root) {
        if(root == null){
            return null;
        }
        TreeNode temp = root.left;
        root.left = root.right;
        root.right = temp;
        if(root.left != null){
            invertTree(root.left);
        }
        if(root.right != null){
            invertTree(root.right);
        }
        return root;
    }
}

解法二：輔助棧

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

class Solution {
    public TreeNode invertTree(TreeNode root) {
        if(root == null){
            return null;
        }
        Deque<TreeNode> stack = new ArrayDeque<TreeNode>();
        stack.addLast(root);
        while(!stack.isEmpty()){
            TreeNode node = stack.removeLast();
            TreeNode temp = node.left;
            node.left = node.right;
            node.right = temp;
            if(node.left != null){
                stack.addLast(node.left);
            }
            if(node.right != null){
                stack.addLast(node.right);
            }
            // 寫在上處也可以
            // TreeNode temp = node.left;
            // node.left = node.right;
            // node.right = temp;
        }
        return root;
    }
}