leetcode_Two Sum II

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

思路1： target-element然后二分查找 O(nlogn)

class Solution {    public int[] twoSum(int[] numbers, int target) {        int[] result = new int[2];        for(int i = 0; i < numbers.length; ++i)        {            int key = target-numbers[i];            int index2 = binarySearch(numbers, key);            //按题意要求，同样的下标不能重复使用            if(index2  != -1 && i != index2)            {                result[0] = i < index2? i+1:index2+1;                result[1] = i <index2?index2+1:i+1;                break;            }        }        return result;    }    private int binarySearch(final int[] numbers, int key)    {        int low = 0;        int hi = numbers.length-1;        while(low <= hi)        {            int mid = (hi+low)/2;            if (numbers[mid] < key) {                low = mid+1;            }            else if(numbers[mid] > key)            {                hi = mid-1;            }            else                 return mid;        }        return -1;    }}public class leetcode_TwoSumIIInputArrayIsSorted {    public static void main(String[] args) {        // TODO Auto-generated method stub        Solution solution = new Solution();        int[] numbers ={1,2, 3,4,4,9,56,90};        int[] result = solution.twoSum(numbers, 8);        for (int i = 0; i < result.length; i++) {            System.out.println(result[i]);        }    }}

但由于题目所提示的是所给的测试用例中，每个测试用例只有唯一的答案！因此该题还可以用下面的思路来优化。

思路2：首尾两个指针，尾指针从后往前移动，首指针从前往后移动，缩小范围 O(n)

class Solution2 {    public int[] twoSum(int[] numbers, int target) {        int[] result = new int[2];        int left = 0;        int right = numbers.length-1;        for(int i = 0; i < numbers.length; ++i)        {            int sum = numbers[left]+numbers[right];            if(sum == target)            {                result[0] = left+1;                result[1] = right+1;                break;            }            else            {                if (sum < target)                {                    left++;                }                else                    right --;            }        }        return result;    }}public class leetcode_TwoSumIIInputArrayIsSorted {    public static void main(String[] args) {        // TODO Auto-generated method stub        Solution2 solution = new Solution2();        int[] numbers ={1,2, 3,4,4,9,56,90};        int[] result = solution.twoSum(numbers, 8);        for (int i = 0; i < result.length; i++) {            System.out.println(result[i]);        }    }}