175.Invert Binary Tree-翻转二叉树（容易题）

翻转二叉树

1. 题目

   翻转一棵二叉树

2. 样例

   

3. 挑战

   递归固然可行，能否写个非递归的？

4. 题解

1.递归法

/**
 * Definition of TreeNode:
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left, right;
 *     public TreeNode(int val) {
 *         this.val = val;
 *         this.left = this.right = null;
 *     }
 * }
 */
public class Solution {
    /**
     * @param root: a TreeNode, the root of the binary tree
     * @return: nothing
     */
    public void invertBinaryTree(TreeNode root) {
        if (root == null)
        {
            return;
        }

        TreeNode tmp = root.left;
        root.left = root.right;
        root.right = tmp;

        invertBinaryTree(root.left);
        invertBinaryTree(root.right);
    }
}

2.非递归法
实际就是二叉树的层序遍历，用队列实现。

/**
 * Definition of TreeNode:
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left, right;
 *     public TreeNode(int val) {
 *         this.val = val;
 *         this.left = this.right = null;
 *     }
 * }
 */
public class Solution {
    /**
     * @param root: a TreeNode, the root of the binary tree
     * @return: nothing
     */
    public void invertBinaryTree(TreeNode root) {
        if (root == null)
        {
            return;
        }

        Queue<TreeNode> q = new LinkedList<>();
        q.add(root);

        while (!q.isEmpty()) 
        {
            TreeNode node = q.poll(); 
            TreeNode tmp = node.left;
            node.left = node.right;
            node.right = tmp;
            if (node.left != null) 
            {
                q.add(node.left);
            }
            if (node.right != null)
            {
                q.add(node.right);
            }
        }
    }
}

Last Update 2016.9.8