8.20笔记

75. 颜色分类（不能用内置函数，利用三指针的方法）

    def sortColors(self, nums):
        """
        :type nums: List[int]
        :rtype: None Do not return anything, modify nums in-place instead.
        """
        red,white,blue = 0,0,len(nums)-1
        while blue >= white:
            if nums[white] == 0:
                nums[white],nums[red] = nums[red],nums[white]
                red += 1
                white += 1
            elif nums[white] == 1:
                white += 1
            else:
                nums[blue],nums[white] = nums[white],nums[blue]
                blue -= 1
        return nums

18. 四数之和

class Solution(object):
    def fourSum(self, nums, target):
        """
        :type nums: List[int]
        :type target: int
        :rtype: List[List[int]]
        """
        def findNsum(l, r, target, N, result, results):
            if r - l + 1 < N or target < nums[l]*N or target > nums[r]*N:
                return
            if N == 2:
                while l < r:
                    s = nums[l] + nums[r]
                    if s == target:
                        results.append(result+[nums[l],nums[r]])
                        l += 1
                        while l < r and nums[l] == nums[l-1]:
                            l += 1
                    elif s < target:
                        l += 1
                    else:
                        r -= 1
            else:
                for i in range(l,r+1):
                    if i == l or (i > l and nums[i-1] != nums[i]):
                        findNsum(i+1,r,target-nums[i],N-1,result+[nums[i]],results)
        results = []
        nums.sort()
        findNsum(0,len(nums)-1,target,4,[],results)
        return results