Two Sum II - Input array is sorted

Question

Leetcode #167 is almost same as #1, but why I cannot only add a if condition?

Q: 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]

The sum of 2 and 7 is 9. 
Therefore index1 = 1, index2 = 2.

My code:

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

}

Why I always return null? where is my mistake? How to fix it?

Answer 1

Because the question says array starts from 1 does not mean array starts from 1 in java.If you want to return i,j as non-zero you should go from 1 to length+1 and then inside the conditions you should check indexes as i-1,j-1 or just start from 0 and return i+1,j+1.

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

or you can do,

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

Answer 2

https://leetcode.com/problems/two-sum-ii-input-array-is-sorted/

[question]: 167. Two Sum II - Input array is sorted

Using the two-pointer technique:-

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