[LeetCode] Two Sum II - Input array is sorted

Posted 2020-09-28 immjc

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

利用二分法遍历数组，如果两数之和等于target，则把索引放入结果数组中，如果两数之和小于target，则让左索引+1，如果两数之和大于target，则让右索引-1。

class Solution {
public:
    vector<int> twoSum(vector<int>& numbers, int target) {
        vector<int> res;
        int left = 0, right = numbers.size() - 1;
        while (left < right) {
            if (numbers[left] + numbers[right] == target) {
                res.push_back(left + 1);
                res.push_back(right + 1);
            }
            if (numbers[left] + numbers[right] < target) 
                left++;
            else 
                right--;
        }
        return res;
    }
};
// 3 ms