LeeyCode - 167. Two Sum II - Input array is sorted

題目：

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. Please note that 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 thesame element twice.

Input: numbers={2, 7, 11, 15}, target=9
Output: index1=1, index2=2

思路與步驟：

設兩個下標 head 和 tail，numbers[head] 和 numbers[tail] 相加，如果大於 target，則 tail--；如果小於，則 head++。直到相等。

編程實現：

public class Solution {
    public int[] twoSum(int[] numbers, int target) {
        int[] indices = new int[2];
        int len = numbers.length;
        int head=0, tail=len-1;
        while(head<=tail){
            if(numbers[head]+numbers[tail] == target){
                indices[0] = head+1;
                indices[1] = tail+1;
                break;
            } else if(numbers[head]+numbers[tail] > target)    tail--;
            else    head++;
        }
        return indices;
    }
}