在一个数组中找到几个数之和为某个数字

思路一: for循环嵌套遍历(想作死可以试下)
思路二: 双指针(分别指向头、尾,往之间收缩)
思路三: hashmap(用空间换时间)

1. 两个数字之和为某个数

# 思路三
def getRes_HashMap(nums, target):
	nums.sort()
    result = set()
    for index, number in enumerate(nums):
        if target-number in nums[index+1:]:
            result.add((number, target-number))
    return result

# 思路二: 使用双指针, 复杂度nlogn(排序)+n（遍历）找到和为target的两个数
def getRes_HashMap(nums, target):
    result = set()
    nums = sorted(nums)
    min_index = 0
    max_index = len(nums)-1
    while min_index < max_index:
        if nums[min_index] + nums[max_index] == target:
            result.add((nums[min_index], nums[max_index]))
            min_index += 1
            max_index -= 1
        elif nums[min_index] + nums[max_index] < target:
            min_index += 1
        else:
            max_index -= 1
    return sorted(list(result))

2. 和为target的两个数并且乘积最小

def getRes_HashMap(nums, target):
	nums.sort()
    result = []
    for index, number in enumerate(nums):
        if target-number in nums[index+1:]:
            if len(result) == 0:
                result.append(number)
                result.append(target-number)
            elif number * (target-number) < result[0] * result[1]:
                result.clear()
                result.append(number)
                result.append(target-number)
    return result

3. 找到和为target的三个数

def getRes_HashMap(nums, target):
    result = set()
    for i, n1 in enumerate(nums):
        for j, n2 in enumerate(nums[i+1:]):
            if target - n1 - n2 in nums[i+j+2:]:
                min_n = min(n1, n2, target-n1-n2)
                max_n = max(n1, n2, target-n1-n2)
                result.add((min_n, target-min_n-max_n, max_n))
    return result

# 使用双指针，找到和为target的三个数
def getRes_HashMap(nums, target):
    result = set()
    nums.sort()
    for i in range(len(nums) - 2):
        min_index = i+1
        max_index = len(nums) - 1
        while min_index < max_index:
            if nums[i] + nums[min_index] + nums[max_index] == target and (nums[i], nums[min_index], nums[max_index]) not in result:
                result.add((nums[i], nums[min_index], nums[max_index]))
                min_index += 1
                max_index -= 1
            elif nums[i] + nums[min_index] + nums[max_index] < target:
                min_index += 1
            elif nums[i] + nums[min_index] + nums[max_index] > target:
                max_index -= 1
    return sorted(list(result))

4. 找到和为target的四个数

# 双指针
def getRes_HashMap(nums, target):
    result = set()
    nums.sort()
    for i in range(len(nums) - 3):
        for j in range(i + 1, len(nums) - 2):
            min_index = j + 1
            max_index = len(nums) - 1
            while min_index < max_index:
                if nums[i] + nums[j] + nums[min_index] + nums[max_index] == target and (nums[i], nums[j], nums[min_index], nums[max_index]) not in result:
                    result.add((nums[i], nums[j], nums[min_index], nums[max_index]))
                    min_index += 1
                    max_index -= 1
                elif nums[i] + nums[j] + nums[min_index] + nums[max_index] < target:
                    min_index += 1
                elif nums[i] + nums[j] + nums[min_index] + nums[max_index] > target:
                    max_index -= 1
    return sorted(list(result))

# map的方式
def getRes_HashMap(nums, target):
    nums.sort()
    result = set()
    for i, n1 in enumerate(nums):
        for j, n2 in enumerate(nums[i+1:]):
            for m, n3 in enumerate(nums[i+j+2:]):
                if target-nums[i]-nums[i+j+1]-nums[i+j+m+2] in nums[i+j+m+3:]:
                    temp = sorted(list([nums[i], nums[i+j+1], nums[i+j+m+2], target-nums[i]-nums[i+j+1]-nums[i+j+m+2]]))
                    result.add((temp[0], temp[1], temp[2], temp[3]))
    return sorted(result)