LeetCode题目：两数之和2

Posted 2021-11-07 zhangchuan1001

题目描述：

给定一个整数数组 nums 和一个目标值 target，请你在该数组中找出和为目标值的那 两个 整数，并返回他们的数组下标。

你可以假设每种输入只会对应一个答案。但是，你不能重复利用这个数组中同样的元素。

示例:

给定 nums = [2, 7, 11, 15], target = 9

因为 nums[0] + nums[1] = 2 + 7 = 9
所以返回 [0, 1]


解法1：
class Solution {
    public int[] twoSum(int[] numbers, int target) {
       int i = 0,j = numbers.length-1;
        int[] nums = new int[2];
        while(i<j)
        {
            if(numbers[i] + numbers[j] < target)
            {
                i++;
            }
            else if(numbers[i] + numbers[j] > target)
            {
                j--;
            }
            else if(numbers[i] + numbers[j] == target)
            {
                nums[0] = i+1;
                nums[1] = j+1;
                break;
              }
        }
        return nums;
    }
}

解法2：二分法

private int binarySearch(int[] arr, int L, int R, int target)
{
    while (L <= R)
    {
        int mid = L + (r - R) / 2;
        if (arr[mid] > target)
        {
            R = mid - 1;
        }
        else if (arr[mid] < target)
        {
            L = mid + 1;
        }
        else
        {
            return mid;
        }
    }
    return -1;
}

public int[] twoSum_2(int[] numbers, int target) {
    for (int i = 0; i <= numbers.length - 2; i++) {
        int cur  = numbers[i];
        int find = target - cur;
        int findIndex = binarySearch(numbers, i + 1, numbers.length - 1, find);
        if (findIndex != -1)
        {
            return new int[] {i + 1, findIndex + 1};
        }
    }
    return null;
}