LeetCode 167. Two Sum II - Input array is sorted 题解（C++）

LeetCode 167. Two Sum II - Input array is sorted 题解（C++）

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题目描述

• 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.

示例

• Input:
  numbers={2, 7, 11, 15}, target=9

• Output:
  index1=1, index2=2

思路

• 这里给的整数数组是按照升序排列的，所以可以利用升序的性质，用O（n）的时间复杂度完成。
• 这里首先用两个指针分别指向数组的第一个元素和最后一个元素，之后将指针指向的两个元素相加并与target作比较，若相等，则得到结果存到indices中并退出循环；否则若两者相加之和大于target，则将第二个指针向前移动一位，若两者相加之和小于target，则将第一个指针向后移动一位。
• 这里返回的indices的位置需要加1，因为题目要求数组下标不能从0开始。

代码

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