Leetcode题解（32）

107. Binary Tree Level Order Traversal II

题目

直接代码：

/**
 * 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:
    vector<vector<int>> levelOrderBottom(TreeNode* root) {
       vector<vector<int>> res;
       vector<int> temp;
       if(NULL == root)
           return res;

       queue<TreeNode*> myQue;
       TreeNode *temp1;
       bool flag = true;
       myQue.push(root);
      
       while (!myQue.empty())
       {
           temp.clear();
           myQue.push(NULL);
           temp1 = myQue.front();
           myQue.pop();
           while (NULL != temp1)
           {
               temp.push_back(temp1->val);
               if(NULL != temp1->left)
                   myQue.push(temp1->left);
               if(NULL != temp1->right)
                   myQue.push(temp1->right);
               temp1 = myQue.front();
               myQue.pop();
           }
          

           res.push_back(temp);
       }
       reverse(res.begin(),res.end());
       return res;
    }
    
};

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108. Convert Sorted Array to Binary Search Tree

题目

分析：

讲一个排好序的数组，构造成平衡二叉树，并且是二叉搜索树，其基本思想是折半（二分）法。

代码如下：

/**
 * 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* sortedArrayToBST(vector<int>& nums) {
        int size = nums.size();
        if(0 == size)
            return NULL;
        TreeNode *root = mySortedArrayToBST(nums,0,size-1);
        return root;
    }
    TreeNode* mySortedArrayToBST(vector<int>& nums,int start,int end)
    {
        int middle = (start+end)/2;
        TreeNode *temp = new TreeNode(nums[middle]);
        if(middle == start)
            temp->left = NULL;
        else
            temp->left = mySortedArrayToBST(nums,start,middle-1);
        
        if(middle == end)
            temp->right = NULL;
        else
            temp->right = mySortedArrayToBST(nums,middle+1,end);
        
        return temp;
    }
};