Description:
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].

My Solution：

class Solution {
    public int[] twoSum(int[] nums, int target) {
        for(int i = 0;i < nums.length;i++){
            for(int j = i + 1;j<nums.length;j++){
                if((nums[i] + nums[j]) == target){
                    return new int[]{i,j};
                }
            }            
        }
        
 
throw new IllegalArgumentException("no two sum solution!");
    }
}

Better Solution:

class Solution {
    //用hashMap存儲{target - nums[i]:i},如果包含target-nums[i]這個key，說明找到兩元素之和為target
    public int[] twoSum(int[] nums, int target) {
        Map<Integer, Integer> map = new HashMap<>();
        
 
for(int i = 0; i < nums.length; i++) {
            if(map.containsKey(nums[i])) {
                return new int[]{map.get(nums[i]), i};
            } 
            map.put(target - nums[i], i);
        }
        return new int[2];
    }
}

leetCode-Two Sum