算法系列——Two Sum II

题目描述

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 the same element twice.

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

解题思路

这道题目要求 在排好序的数组numbers中,找到 和为target 的两个元素，返回这两个元素的索引。 采用 对撞指针的方法是一种时间复杂度为O(n)的最优解法。

特别注意：索引值从1开始。

算法实现

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