二分查找

经典二分查找，给定一个升序的整形数组nums和一个目标值target，查找target在nums中的位置，如果目标值存在返回下标，否则返回-1

public class Solution {
    public int Search(int[] nums, int target) {
        return BinarySearch(nums, 0, nums.Length - 1, target);
    }

    public int BinarySearch(int[] nums, int L, int R, int target)
    {
        while (L <= R)
        {
            int mid = L + (R - L) / 2;
            if (nums[mid] == target)
                return mid;
            else if (nums[mid] < target)
                L = mid + 1;
            else
                R = mid - 1;
        }

        return -1;
    }
}

二分查找的一个典型扩展: 给定一个升序数组和目标值，在数组中找到目标值并返回其索引，如果目标值不存在，返回它将被顺序插入的位置。只需将上述代码稍作修改即可

public class Solution {
    public int SearchInsert(int[] nums, int target) {
        int left = 0, right = nums.Length - 1;
        while (left <= right)
        {
            int mid = left + (right - left) / 2;
            if (nums[mid] == target)
                return mid;
            else if (nums[mid] > target)
                right = mid - 1;
            else 
                left = mid + 1;
        }

        return right + 1;
        // return left;
    }
}