leetcode-167. Two Sum II - Input array is sorted

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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.

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算法设计：

定义两个索引 i=0、j=numbers.size()-1，当 i<j 时进行循环并进行以下判断：

1. 如果numbers[i]+numbers[j]>target，令 j 左移一位，--j
2. 如果numbers[i]+numbers[j]<target，令 i 右移一位，++i
3. 如果numbers[i]+numbers[j]==target，说明找到了一组符合要求的两个数，输出，退出循环

C++代码：

class Solution {
public:
    vector<int> twoSum(vector<int>& numbers, int target) {
        for(int i=0,j=numbers.size()-1;i<j;){
            int temp=numbers[i]+numbers[j];
            if(temp==target)
                return {i+1,j+1};
            if(temp<target)
                ++i;
            else
                --j;
        }
    }
};