167. Two Sum II - Input Array Is Sorted

问题描述
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.

由于是有序数列，因此可以采用双指针。
左右两侧和不等于目标时，根据大小结果移动左右指针。

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