Question

Formatted question description: https://leetcode.ca/all/167.html

 167	Two Sum II - Input array is sorted

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

Algorithm

We only need two pointers, one pointing to the beginning, one pointing to the end, and then traversing toward the middle.

  • If the two numbers pointed to are exactly equal to the target, the position of the two pointers can be directly returned.
  • If it is less than the target, the left pointer is right. Move one bit, if greater than target, the right pointer moves one bit to the left, and so on until the two pointers meet and stop

Code

Java

public class Two_Sum_II_Input_array_is_sorted {

    // time: O(N)
    // space: O(1)
    class Solution {
        public int[] twoSum(int[] numbers, int target) {
            int i = 0, j = numbers.length-1;
            int sum;
            while (i < j) {
                sum = numbers[i] + numbers[j];
                if (sum == target)
                    return new int[]{i+1, j+1};
                else if (sum < target)
                    i++;
                else
                    j--;
            }

            return null;

        }
    }
}

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