226. Invert Binary Tree

Invert a binary tree.

     4
   /   
  2     7
 /    / 
1   3 6   9

to

     4
   /   
  7     2
 /    / 
9   6 3   1


反转一颗二叉树

c++（3ms）：

/**
 * 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){
            invertTree(root->left) ;
            invertTree(root->right) ;
            swap(root->left , root->right) ;
        }
        return root ;
    }
};

C++(3ms):

/**
 * 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) {
        stack<TreeNode*> st ;
        st.push(root) ;
        while(!st.empty()){
            TreeNode* t = st.top() ;
            st.pop() ;
            if (t){
                st.push(t->left) ;
                st.push(t->right) ;
                swap(t->left,t->right) ;
            }
        }
        return root ;
    }
};