leetcode解题: Two Sum II - Input array is sorted (167)

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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

解法1：O(N) Time + O(1) Complexity

如果是已经sorted的话这题就很简单了，比较容易的能想到可以用two pointers， left和right按照加和的大小不停的移动。

Java

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