Given an array of integers that is already sorted in ascending order, find two numbers such that they add up to a specific target number.

The function twoSum should return indices of the two numbers such that they add up to the target, where index1 must be less than index2. Please note that your returned answers (both index1 and index2) are not zero-based.

You may assume that each input would have exactly one solution and you may not use the same element twice.

Input: numbers={2, 7, 11, 15}, target=9
Output: index1=1, index2=2

分析

一个已排序数组，寻找两个数字的和为指定值。很简单的二分查找即可实现，代码如下。

/**
 * Return an array of size *returnSize.
 * Note: The returned array must be malloced, assume caller calls free().
 */
int* twoSum(int* numbers, int numbersSize, int target, int* returnSize) {
    int index1=0,index2=numbersSize-1;
    while(index1<index2)
    {
        if(numbers[index1]+numbers[index2]==target)
            break;
        else if(numbers[index1]+numbers[index2]<target)
        {
            index1++;
        }
        else
            index2--;
    }
    int *ans=(int*)malloc(sizeof(int)*2);
    ans[0]=index1+1;
    ans[1]=index2+1;
    *returnSize=2;
    return ans;
}