LeetCode Two Sum

原题链接在这里：https://leetcode.com/problems/two-sum/

题目：

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.

Example:

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

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

题解：

采用HashMap记录之前出现的num 和 其对应的 index.

Time Complexity: O(n). Space: O(n).

AC Java:

 1 public class Solution {
 2     public int[] twoSum(int[] nums, int target) {
 3         if(nums == null || nums.length < 2){
 4             throw new IllegalArgumentException("Invalid input.");
 5         }
 6         
 7         int [] res = {-1, -1};
 8         HashMap<Integer, Integer> hm = new HashMap<Integer, Integer>();
 9         for(int i = 0; i<nums.length; i++){
10             if(!hm.containsKey(target - nums[i])){
11                 hm.put(nums[i], i);
12             }else{
13                 res[0] = hm.get(target-nums[i]);
14                 res[1] = i;
15                 break;
16             }
17         }
18         
19         return res;
20     }
21 }

 后面跟上Two Sum II - Input array is sorted, Two Sum III - Data structure design, Two Sum IV - Input is a BST, 3Sum, 3Sum Closest, 4Sum.