迭代 Flatten Binary Tree to Linked List

思路：

迭代。

按照Pre-Order的顺序不断链接在右子树上。

/**
 * 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:
    void flatten(TreeNode* root) {
        if(root == NULL) return;
        stack<TreeNode*> s;
        s.push(root);
        while(!s.empty()) {
            TreeNode *node = s.top();
            s.pop();
            if(node->right != NULL) {
                s.push(node->right);
            }
            if(node->left != NULL) {
                s.push(node->left);
            }
            node->left = NULL;
            if(!s.empty()) {
                node->right = s.top();
            }
        }
    }
};