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

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Problem

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:

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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.

Explanation

  1. Use two pointers technique. Check most left and most right poitners first. If the sum is smaller than target, then move left poitner one step to the right; else if the sum is larger, then move right pointer one step to the left; else return the result index.

Solution

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class Solution {
    public int[] twoSum(int[] numbers, int target) {
        int left = 0, right = numbers.length-1;
        while (left < right) {
            int sum = numbers[left] + numbers[right];
            if (sum == target) return new int[] {left+1, right+1};
            else if (sum < target) left += 1;
            else right -= 1;
        }
        return new int[2];
    }
}