leetcode 二叉树系列

1.二叉树转链表

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

class Solution {
public:
    vector<TreeNode *>v;
    void flatten(TreeNode *root) 
    {
      v.clear();
      if(root==NULL)return;
      PreOrder(root);
      TreeNode *p=root;
      for(int i=1;i<v.size();i++)
      {
          TreeNode *q=v[i];
          p->left=NULL;
          p->right=q;
          p=q;
      }       
    }
    void PreOrder(TreeNode *root)
    {
       if(root)v.push_back(root);
       if(root->left)PreOrder(root->left);
       if(root->right)PreOrder(root->right);
    }
};

 2.数组建立平衡二叉树

Convert Sorted Array to Binary Search Tree

Given an array where elements are sorted in ascending order, convert it to a height balanced BST.

此方法非常巧妙，取中点作为根节点，递归建立左右子树

    class Solution {
    public:
        TreeNode *sortedArrayToBST(vector<int> &num) {
            return _sortArrayToBST(num, 0, num.size()-1);
        }
    private:
        TreeNode *_sortArrayToBST(vector<int> &num, int i, int j){
            if(i>j){
                return NULL;
            }
            else{
                int r = (i+j+1)/2;
                TreeNode *root = new TreeNode(num[r]);
                root->left = _sortArrayToBST(num, i, r-1);
                root->right = _sortArrayToBST(num, r+1, j);
                return root;
            }       
        }
    };