Q:

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].

S1：Time：O（n2） Space:O(1)

func twoSum(_ nums: [Int], _ target: Int) -> [Int] {
    for var i in 0..<nums.count {
        let first = nums[i];
        if first > target {
            return [];
        }
        let addMore = target - first;
        for var j in
 
 i+1..<nums.count {
            let second = nums[j];
            if second > addMore {
                break;
            }
            if second == addMore {
                return [i,j];
            }
        }
    }

    return [];
}

S2：Time：O（n） Space:O(n)

func twoSum(_ nums: [Int], _ target: Int)
 
 -> [Int] {
    var dic = [Int:Int]();
    for var i in 0..<nums.count {
        let item = nums[i];
        let addMore = target - item;
        let dicItem = dic[addMore];
        if  dicItem != nil {
            return [dicItem!,i];
        }
        dic[item] = i;

    }
    return [];
}