LeetCode 1. Two Sum

Question

Given an array of integers, return indices of the two numbers such that they add up to a specific target.

You may assume that each input would have exactly one solution, and you may not use the same element twice.

Example:
Given nums = [2, 7, 11, 15], target = 9,

Because nums[0] + nums[1] = 2 + 7 = 9,
return [0, 1].

Explanation

基础中的基础题。如果用Hashmap存，就要用额外的空间，如果用排序和双指针来做，则需要O（nlogn）时间复杂度。

Code

public class Solution { public int[] twoSum(int[] nums, int target) { int[] result = new int[2]; HashMap<Integer, Integer> map = new HashMap<>(); for (int i= 0; i< nums.length; i++) { if (map.containsKey(nums[i])) { result[0] = map.get(nums[i]); result[1] = i; break; } else { map.put(target - nums[i] , i); } } return result;

}

}