226. 翻转二叉树

翻转一棵二叉树。

Solution1（递归版）：

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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) return NULL;
        
        if(root->left) invertTree(root->left);
        if(root->right) invertTree(root->right);
        TreeNode* temp = root->left;
        root->left = root->right;
        root->right = temp;
        return root;
    }
};

依次从叶子结点向上翻转。

Solution2（层次遍历版）：

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class Solution {
public:
    TreeNode* invertTree(TreeNode* root) {
        if(!root) return NULL;
        queue<TreeNode*> que;
        que.push(root);
        while(!que.empty()){
            TreeNode* node = que.front();
            que.pop();
            TreeNode* nodeleft = node->left;
            node->left = node->right;
            node->right = nodeleft;
            if(node->left) que.push(node->left);
            if(node->right) que.push(node->right);
        }
        return root;
    }
};

层次遍历，每到一个结点，就讲该结点下的子结点交换。