InvertBinaryTree.cpp

// Source : https://leetcode.com/problems/invert-binary-tree/
// Author : Hao Chen
// Date   : 2015-06-12

/********************************************************************************** 
 * 
 * Invert a binary tree.
 *      4
 *    /   \
 *   2     7
 *  / \   / \
 * 1   3 6   9
 * 
 * to
 *      4
 *    /   \
 *   7     2
 *  / \   / \
 * 9   6 3   1
 * 
 * Trivia:
 * This problem was inspired by this original tweet by Max Howell:
 * (https://twitter.com/mxcl/status/608682016205344768)
 *
 *  | Google: 90% of our engineers use the software you wrote (Homebrew), 
 *  | but you can’t invert a binary tree on a whiteboard so fuck off.
 *               
 **********************************************************************************/

/**
 * 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_recursive(TreeNode* root) {
        if (root==NULL) return root;
        TreeNode* node = invertTree_recursive(root->left);
        root->left = invertTree_recursive(root->right);
        root->right = node;
        return root;
    }
    
    TreeNode* invertTree_non_recursive(TreeNode* root) {
        if (root==NULL) return root;
        vector<TreeNode*> stack;
        stack.push_back(root);
        while (!stack.empty()) {
            TreeNode* node = stack.back();
            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;
    }
    
    TreeNode* invertTree(TreeNode* root) {
        if (rand()%2){
            return invertTree_non_recursive(root);
        }
        return invertTree_recursive(root);    
    }
};