34. 在排序数组中查找元素的第一个和最后一个位置

思路：
与AcWing 789一致

class Solution {
public:
    vector<int> searchRange(vector<int>& nums, int target) {
        if (nums.size() == 0) return {-1, -1};
        int begin, end;
        int l = 0, r = nums.size() - 1;
        while (l < r) {
            int mid = l + r >> 1;
            if (nums[mid] >= target) r = mid;
            else l = mid + 1;
        } 
        if (nums[l] != target) return {-1, -1};
        else {
            begin = l;
            int l = 0, r = nums.size() - 1;
            while (l < r) {
                int mid = l + r + 1 >> 1;
                if (nums[mid] <= target) l = mid;
                else r = mid - 1;
            } 
            end = l;
        }
        return {begin, end};
    }
};