Question
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
Solution
To find 2 sums in a sorted array, we could use a bi-dementioanl search. Set two indecies start and end for the start and end of the search range and compare sum of nums[start] + nums[end] and 0 - nums[first]. When sum == 0 - nums[first], we know a valid 2 sum is found. When sum < 0 - nums[first], increase sum by moving start forward. When sum > 0 - nums[first], decrease sum by moving end backwards.
Code
class Solution {
public int[] twoSum(int[] numbers, int target) {
int l = 0;
int r = numbers.length-1;
while(l<r) {
if(numbers[l] + numbers[r] == target) {
return new int[] {l+1, r+1};
}
else if(numbers[l] + numbers[r] < target) {
l++;
}
else {
r--;
}
}
return null;
}
}
Performance
O(n) for both time and space complexity.