【easy】226. Invert Binary Tree 反转二叉树

/**
 * 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 NULL;
        }

        TreeNode *tempNode = root->left;
        root->left = root->right;
        root->right = tempNode;

        invertTree(root->left);
        invertTree(root->right);

        return root;
    }
};

 下面是非递归的一种写法：（可以说是很漂亮了）

TreeNode* invertTree(TreeNode* root)   
   {  
       if(root == NULL)return NULL;  
       vector<TreeNode*> stack;  
       stack.push_back(root);  
       while(!stack.empty())  
       {  
           TreeNode* node = stack.back();// or stack.top()  
           stack.pop_back();  
           swap(node->left,node->right);  
           if(node->left)stack.push_back(node->left);  
           if(node->right)stack.push_back(node->right);  
       }  
       return root;  
   }