167. Two Sum II - Input Array Is Sorted

Problem

Leetcode#167

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.


Example 1:
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. We return [1, 2].

Example 2:
Input: numbers = [2,3,4], target = 6
Output: [1,3]
Explanation: The sum of 2 and 4 is 6. Therefore index1 = 1, index2 = 3. We return [1, 3].

Example 3:
Input: numbers = [-1,0], target = -1
Output: [1,2]
Explanation: The sum of -1 and 0 is -1. Therefore index1 = 1, index2 = 2. We return [1, 2].

Constraints:
2 <= numbers.length <= 3 * 104
-1000 <= numbers[i] <= 1000
numbers is sorted in non-decreasing order.
-1000 <= target <= 1000
The tests are generated such that there is exactly one solution.

Approach

• Đề bài cho là một mảng không giảm và đánh index từ 1
• Bài này, chỉ đơn giản dùng 2 pointer, pointer left p_left và pointer right p_right lần lượt duyệt từ trái và phải của mảng hướng vào giữa mảng
• Nếu tổng s = numbers[p_left] + numbers[p_right]
  • s = target thì (p_left+1, p_right+1) là 2 index cần tìm (đề bài cho mảng đánh từ index 1)
  • s > target thì giảm p_right 1 index p_right--, vì mảng đã sắp xếp không giảm
  • s < target thì tăng p_left 1 index p_right++

Implementation

class Solution {
    public int[] twoSum(int[] numbers, int target) {
        int p_left = 0; int p_right = numbers.length-1;

        while (p_left < p_right) {
            int leftVal = numbers[p_left]; int rightVal = numbers[p_right];
            if (leftVal + rightVal == target) {
                return new int[] {p_left+1, p_right+1};
            } else if (leftVal + rightVal > target) {
                p_right--;
            } else {
                p_left++;
            }
        }
        return new int[]{};
    }
}