Array：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

解答

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

分析

用两个指针，一个指向数组头，一个指向数组尾，由于数组是升序的，如果两个指针指向的数字之和大于target，将将右指针左移一位，反之将左指针右移一位，知道等于target或两指针相遇。