二分查找应用2
2021-09-19 / ryanxw

题目

给定一个按照升序排列的整数数组 nums，和一个目标值 target。找出给定目标值在数组中的开始位置和结束位置。如果数组中不存在目标值 target，返回 [-1, -1]。

  • 要求：时间复杂度为 O(log n)
  • 例子：
    • 输入：nums = [5,7,7,8,8,10], target = 8 输出：[3,4]
    • 输入：nums = [5,7,7,8,8,10], target = 6 输出：[-1,-1]
  • leetcode链接

解法：改写二分

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int FindLeft(vector<int>& nums, int target) {
    int l = 0;
    int r = nums.size() - 1;
    int left = -1;
    while (l <= r){
        int mid = l + (r - l) / 2;
        if (nums[mid] == target){
            left = mid;
            r = mid - 1;
        }else if (nums[mid] < target){
            l = mid + 1;
        }else{
            r = mid - 1;
        }
    }
    return left;
}

int FindRight(vector<int>& nums, int target) {
    int l = 0;
    int r = nums.size() - 1;
    int right = -1;
    while (l <= r){
        int mid = l + (r - l) / 2;
        if (nums[mid] == target){
            right = mid;
            l = mid + 1;
        }else if (nums[mid] < target){
            l = mid + 1;
        }else{
            r = mid - 1;
        }
    }
    return right;
}

vector<int> searchRange(vector<int>& nums, int target) {
    vector<int> ans;
    int l = FindLeft(nums, target);
    int r = FindRight(nums, target);
    ans.push_back(l);
    ans.push_back(r);
    return ans;
}