LeetCode 167. Two Sum II - Input array is sorted

题目

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.

Note:

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.
Example:

Input: numbers = [2,7,11,15], target = 9
Output: [1,2]
Explanation: The sum of 2 and 7 is 9. Therefore index1 = 1, index2 = 2.

解析

认真读题，题目中说两个数加起来等于target，并且这两个数在有序数组中是一定存在的，有唯一解。于是，这题就变的简单了。
使用两个指针，一个在左侧，一个在右侧，依次向前，如果两个数的和等于target，即为目标解；如果大于，则--right；小于，则++left。最终找到目标解。

代码（C语言）

int* twoSum(int* numbers, int numbersSize, int target, int* returnSize) {
    if (numbers == NULL || numbersSize == 1) {
        (* returnSize) = 0;
        
        return NULL;
    }
    
    int left = 0, right = numbersSize - 1;
    
    int* returnArr = (int*)malloc(sizeof(int) * 2);
    (* returnSize) = 2;
    
    while (left < right) {
        if ((numbers[left] + numbers[right]) == target) {
            returnArr[0] = left + 1;
            returnArr[1] = right + 1;
            
            return returnArr;
        } else if ((numbers[left] + numbers[right]) > target) {
            --right;
        } else
            ++left;
    }
    
    return NULL;
}