[leetcode] flatten binary tree to linked list

Flatten Binary Tree to Linked List

Given a binary tree, flatten it to a linked list in-place.

For example,
Given

         1
        / \
       2   5
      / \   \
     3   4   6

The flattened tree should look like:

   1
    \
     2
      \
       3
        \
         4
          \
           5
            \
             6

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    void flatten(TreeNode *root) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
       
        // use a 'pre' pointer to track the predecessor of the current node
        // use pre-order traversal
       
        if (!root)
            return;
       
        stack my_stack;
        TreeNode *pre = NULL;
       
        my_stack.push(root);
        while (!my_stack.empty())
        {
            TreeNode *p = my_stack.top();
           
            // visit p
            if (pre)
            {
                pre->right = p;
                pre->left = NULL;
            }
           
            pre = p;
           
            // pop current node
            my_stack.pop();
           
            // push right and left children into stack
            if (p->right) my_stack.push(p->right);
            if (p->left) my_stack.push(p->left);
        }
    }
};