Leetcode解题: Invert Binary Tree (226)

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invert a binary tree.

4

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

解法1：Recursion, Divide & Conquer, O(N) time

很典型的一个Divide & Conquer题目，用递归的办法很简单。先对左子树invert，然后对右子树invert,最后交换左右子树即可。
C++

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/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    TreeNode* invertTree(TreeNode* root) {
        if (root == NULL) {
            return root;
        }
        invertTree(root->left);
        invertTree(root->right);
        TreeNode* temp = root->left;
        root->left = root->right;
        root->right = temp;
        return root;
    }
};

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