001 Two Sum 两个数的和为目标数字

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].
详见：https://leetcode.com/problems/two-sum/description/

给定一个整数数组，找出其中两个数满足相加等于你指定的目标数字。

要求：这个函数twoSum必须要返回能够相加等于目标数字的两个数的索引，且index1必须要小于index2。

Java实现：

暴力解：

class Solution {
    public int[] twoSum(int[] nums, int target) {
        int[] res=new int[2];
        for(int i=0;i<nums.length;++i){
            for(int j=i+1;j<nums.length;++j){
                if(nums[i]+nums[j]==target){
                    res[0]=i;
                    res[1]=j;
                }
            }
        }
        return res;
    }
}

Map：空间换时间

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

python：

方法一：

class Solution:
    def twoSum(self, nums: List[int], target: int) -> List[int]:
        hash={}
        for i in range(len(nums)):
            if target-nums[i] in hash:
                return hash[target-nums[i]],i
            else:
                hash[nums[i]]=i
        return -1,-1

方法二：

class Solution:
    def twoSum(self, nums: List[int], target: int) -> List[int]:
        for i in range(len(nums)):
            if target-nums[i] in nums and nums.index(target-nums[i])!=i:
                return nums.index(target-nums[i]),i
        return -1,-1

golang:

方法一：

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

方法二：

func twoSum(nums []int, target int) []int {
    hash:=make(map[int]int)
    for i,v:=range nums{
        j,ok:=hash[target-v]
        if ok{
            return []int{i,j}
        }
        hash[v]=i
    }
    return []int{}
}