1. Two Sum.md

1. Two Sum

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.

Example:

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

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

tag:

Solution

方法一：暴力破解

时间复杂度O(N²)

竟然Accepted :)

方法二：哈希表

时间复杂度O(N)， 空间复杂度O(N)；

java

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

go

func twoSum(nums []int, target int) (int, int) {
	sort.Ints(nums)
	for i, j := 0, len(nums)-1; i < j; {
		if nums[i]+nums[j] == target {
			return i, j
		} else if nums[i]+nums[j] < target {
			i++
		} else {
			j--
		}
	}
	return 0, len(nums) - 1
}

方法三：排序，逼近

时间复杂度O(NLogN)