1-Two-Sum.md

1-Two Sum (Java)

Problem

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].

Solution 1.

此方法最直覺用雙迴圈下去一序做比較。

• 暴力、窮舉
• Run Time: 41 ms
• 時間複雜度: O(n²)
• 空間複雜度: O(1)

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){
                    int arr[]={i,j};
                    return arr; 
                }
            }
        }
		return nums;
    }
}

Solution 2.

此種方法是利用容器 HashMap 下去實作使用 Key、Value 下去做搜尋，Key 儲存 twoSum 裡的內容，Value 儲存 twoSum 裡的索引值。

• One-pass Hash Table
• Run Time: 8 ms
• 時間複雜度 O(n)
• 空間複雜度 O(n)

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