twosum

给定一个整数数组和一个目标值，找出数组中和为目标值的两个数。

你可以假设每个输入只对应一种答案，且同样的元素不能被重复利用。

示例:

给定 nums = [2, 7, 11, 15], target = 9

因为 nums[0] + nums[1] = 2 + 7 = 9
所以返回 [0, 1]

方法一：

            class Solution(object):
                    def twoSum(self, nums, target):
                    """
                            :type nums: List[int]
                            :type target: int
                            :rtype: List[int]
                    """
                            for i in range(len(nums)):
                                        diff =target-nums[i]
                                        if diff in nums:
                                        if nums.index(diff)==i: # For nums = [3,3] & target =6, then result will be [0,0] without this statement, hence we need to check and continue if satisfies.
                                                        continue
                                        return [i,nums.index(diff)]

方法二：

                class Solution:
                        def twoSum(self, nums, target):
                                    seen={}
                                    for i,num in enumerate(nums):
                                                correct = target - num
                                                if correct not in seen:
                                                                    seen[num]=i
                                                else:
                                                        return [seen[correct],i]

方法三：

                    def twoSum(self, nums, target):
                    """
                    :type nums: List[int]
                    :type target: int
                    :rtype: List[int]
                     """
                            u=dict()
                            for i in range(len(nums)):
                                    u[nums[i]]=i

                            for i in range(len(nums)):
                                    dif=target-nums[i]
                                    if dif in u and  u[dif]!=i:
                                            return [i, u[dif]]
                            return []