翻转二叉树（递归与非递归）

[code]  1         1
 / \       / \
2   3  => 3   2
   /       \
  4         4

递归版本

[code]void swapTree(TreeNode *&root){
    TreeNode *tmp = root->left;
    root->left = root->right;
    root->right = tmp;
}

void invertBinaryTree(TreeNode *root) {
    // write your code here
    if(root == NULL)
        return;

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

    swapTree(root);
}

非递归版本

[code]void swapTree(TreeNode *&root){
    TreeNode *tmp = root->left;
    root->left = root->right;
    root->right = tmp;
}
void invertBinaryTree(TreeNode *root) {
    // write your code here
    if(root == NULL)
        return;
    stack<TreeNode*> stk;
    stk.push(root);
    while(!stk.empty())
    {
        TreeNode *tmp = stk.top();
        stk.pop();
        swapTree(tmp);
        if(tmp->left)
            stk.push(tmp->left);
        if(tmp->right)
            stk.push(tmp->right);
    }
}