Leetcode(java) 167. Two Sum II - Input Array Is Sorted

Q:

Given a 1-indexed array of integers numbers that is already sorted in non-decreasing order, find two numbers such that they add up to a specific target number. Let these two numbers be numbers[index1] and numbers[index2] where 1 <= index1 < index2 <= numbers.length.

Return the indices of the two numbers, index1 and index2, added by one as an integer array [index1, index2] of length 2.

The tests are generated such that there is exactly one solution. You may not use the same element twice.

Your solution must use only constant extra space.

A: I will use a head pointer and a tail pointer to scan the array.

Since the array is sorted, initially the head pointer points to the smallest number and the tail pointer points to the largest number.

When the head value plus the tail value is samller than target, the head pointer should be added.

When the head value plus the tail value is larger than target, the tail pointer should be decreased.

class Solution {
    public int[] twoSum(int[] numbers, int target) {
        int n = numbers.length;
        int i = 0, j = n-1;
        while(i<j){
            if((numbers[i]+numbers[j])==target){
                return new int[]{i+1,j+1};
            }else if((numbers[i]+numbers[j])>target){
                j--;
            }else{
                i++;
            }
        }
        return null;
    }
}