DW组队学习——Task01：数组

两数之和

使用字典，可以大大提高查找效率。

class Solution(object):
    def twoSum(self, nums, target):
        """
        :type nums: List[int]
        :type target: int
        :rtype: List[int]
        """
        # HashTable -> Dictionary
        hashDic = {}
        i = 0
        while i < len(nums):
            if hashDic.has_key(target-nums[i]):
                return i,hashDic[target-nums[i]]
            hashDic[nums[i]] = i
            i = i+1
        return

删除有序数组中的重复项

这里一开始一直忽略了特殊情况：数组中至少有一个元素不是重复的，所以可以从下标1开始。

class Solution(object):
    def removeDuplicates(self, nums):
        """
        :type nums: List[int]
        :rtype: int
        """
        
        if not nums:
            return 0
        l = r = 1
        while r < len(nums):
            if nums[r] != nums[r-1]:
                nums[l]=nums[r]
                l+=1
            r+=1

        return l

移除元素

这题和上面一题差不多，所以很快就写完了。

class Solution(object):
    def removeElement(self, nums, val):
        """
        :type nums: List[int]
        :type val: int
        :rtype: int
        """
        
        if not nums:
            return 0
        
        index = 0
        for i in range(len(nums)):
            if nums[i]!=val:
                nums[index]=nums[i]
                index+=1
        return index

三数之和

题目有的难度，一时半会没有想出来。参考了力扣的答案：

class Solution(object):
    def threeSum(self, nums):
        """
        :type nums: List[int]
        :rtype: List[List[int]]
        """
        
        n = len(nums)
        nums.sort()
        ans = list()
        
        for first in range(n):
            if first > 0 and nums[first] == nums[first - 1]:
                continue
            third = n - 1
            target = -nums[first]
            for second in range(first + 1, n):
                if second > first + 1 and nums[second] == nums[second - 1]:
                    continue
                while second < third and nums[second] + nums[third] > target:
                    third -= 1
                if second == third:
                    break
                if nums[second] + nums[third] == target:
                    ans.append([nums[first], nums[second], nums[third]])
        
        return ans