leetcode-search-for-a-range

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题目大意

https://leetcode.com/problems/search-for-a-range/

一个排好序的数组中查找给定target元素的起始和结束位置

解题思路

二分查找。找>=target的元素的最小值和<=target元素的最大值

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vector<int> searchRange(vector<int>& nums, int target) {
        int r1, r2;
        vector<int> ret;
        int size = nums.size();
        if(size == 0) {
            ret.push_back(-1);
            ret.push_back(-1);
            return ret;
        }
        if(size == 1) {
            if(nums[0] == target) {
                ret.push_back(0);
                ret.push_back(0);
            } else {
                ret.push_back(-1);
                ret.push_back(-1);
            }
            return ret;
        }
    
        int L = 0;
        int R = size;
        int mid;
        //find the minimum number of >= target
        while(L + 1 < R) {
            mid = (L + R - 1) / 2;
            if(nums[mid] >= target) {
                R = mid + 1;
            } else {
                L = mid + 1;
            }
        }
        if(nums[L] != target) {
            ret.push_back(-1);
            ret.push_back(-1);
            return ret;
        }
        r1 = L;
        L = 0;
        R = size;
        while(L + 1 < R) {
            mid = (L + R) / 2;
            if(nums[mid] <= target) {
                L = mid;
            } else {
                R = mid;
            }
        }
        r2 = L;
        ret.push_back(r1);
        ret.push_back(r2);
        return ret;   
}

[L,R)是可能区间，每次循环以后更新L和R值仍保证此性质