二分查找LeetCode 34 Search for a Range

LeetCode 34

Search for a Range

• Problem Description：
  给出一串升序排列的数组元素和目标元素target，如果在数组中可以找到target，则范围其位置范围；如果找不到则返回[-1, -1]。
  具体的题目信息：
  https://leetcode.com/problems/search-for-a-range/description/
• Solution:

class Solution {
public:
    vector<int> searchRange(vector<int>& nums, int target) {
        vector<int> res;
        if (nums.size() == 0 ) {
            res.push_back(-1);
            res.push_back(-1);
            return res;
        }
        //通过二分查找找到一个目标元素
        int low = 0, high = nums.size()-1, mid, flag = 0;
        while(low <= high) {
            mid = low+(high-low)/2;
            if (target == nums[mid]) {
                flag = 1;
                break;
            } else if (nums[mid]>target) {
                high = mid-1;
            } else {
                low = mid+1;
            }
        }
        if (flag == 0) {
            res.push_back(-1);
            res.push_back(-1);
        } else {
        //在数组中该目标元素位置向左右扩展，得到边界位置信息
            int pos = mid;
            pos = mid-1;
            while(pos >= 0 && nums[pos] == target) {
                pos--;
            }
            int begin = pos+1;
            pos = mid+1;
            while(pos < nums.size()&&nums[pos] == target) {
                pos++;
            }
            int end = pos-1;
            res.push_back(begin);
            res.push_back(end);
        }
        return res;
    }
};