167.Two Sum II - Input array is sorted

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.

分析

对撞指针，题目中的序列为有序数组（从左至右逐渐变大），分别定义两个指针指向数组的两端，计数指针指向求和，若等于target，返回当前指针索引；若小于target，左侧指针向右进一位；若大于target，右侧指针向左退一位。

代码

class Solution {
public:
	vector<int> twoSum(vector<int>& numbers, int target) {
		int l = 0;
		int r = numbers.size() - 1;
		while (l < r)
		{
			if (numbers[l] + numbers[r] == target)
			{
				int res[2] = { l + 1,r + 1 };       //定义数组
				return vector<int>(res, res + 2);   //用数组初始化vector
			}
			else if (numbers[l] + numbers[r] > target)
			{
				r--;
			}
			else
			{
				l++;
			}
		}
		throw runtime_error("input array is wrong");  //未找到结果，抛出异常，输入有误
	}
};