167. Two Sum II - Input Array Is Sorted

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].

Solution

class Solution {
    public int[] twoSum(int[] numbers, int target) {
        int[] result = new int[2];
        int n = numbers.length;
        for (int i = 0; i < n; i++){
            for (int j = i + 1; j < n; j++){
                if (numbers[i] + numbers[j] == target){
                    result[0] = i + 1;
                    result[1] = j + 1;
                    break;
                }
            }
        }
        return result;
    }
}

// TC: O(n^2)
// SC: O(1)

// Time Limit Exceeded

class Solution {
    public int[] twoSum(int[] numbers, int target) {
        int[] result = new int[2];
        int n = numbers.length;
        Map<Integer, Integer> map = new HashMap<>();

        for (int i = 0; i < n; i++){
            int cur = numbers[i];
            int need = target - cur;
            if (map.containsKey(need)){
                result[0] = map.get(need) + 1;
                result[1] = i + 1;
                break;
            }else{
                map.put(cur, i);
            }
        }
        return result;
    }
}

// TC: O(n)
// SC: O(n)

sorted:

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

        return result;

    }
}

// TC: O(n)
// SC: O(1)

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