[面试真题] LeetCode：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

Hints: If you notice carefully in the flattened tree, each node's right child points to the next node of a pre-order traversal.

解法 1：非递归解法

/**
 * 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
        if(NULL == root){
            return;
        }
        stack<TreeNode*> s;
        TreeNode *p = root;
        s.push(root);
        while(p->left){
            s.push(p->left);
            p = p->left;
        }
        while(s.size()){
            TreeNode *cur = s.top();
            s.pop();
            if(cur->right){
                p->left = cur->right;
                cur->right = NULL;
                while(p->left){
                    s.push(p->left);
                    p = p->left;
                }
            }
        }
        p = root;
        while(p->left){
            p->right = p->left;
            p->left = NULL;
            p = p->right;
        }
    }
};

Run Status: Accepted!
Program Runtime: 44 milli secs

Progress: 225/225 test cases passed.