LeetCode 53 分治算法，求最大子串

本来想用C++写的，但是写的过程中发现很多语法都记不清了，索性转回python，c++以后再练吧

问题描述：

Find the contiguous subarray within an array (containing at least one number) which has the largest sum.

For example, given the array

[-2,1,-3,4,-1,2,1,-5,4]

,

the contiguous subarray

[4,-1,2,1]

has the largest sum =

6

.

解题思路：采取分治的方法，数组分为两部分s1[:len/2],s2[len/2:]

那么只有可能出现一下三种情况：

1、maxsum 在s1中

2、maxsum 在s2中

3、maxsum 跨越s1和s2，那么maxsum(s) = maxsum(s1)+maxsum(s2)

代码：

class Solution(object):

    def maxSubArray(self, nums):

        """

        :type nums: List[int]

        :rtype: int

        """

        sum = 0

         

        l = len(nums)

        if l == 1:

            return nums[0]

         

        left = self.maxSubArray(nums[:l//2])

        right = self.maxSubArray(nums[l//2:])

         

        s1 = nums[(l//2)-1]

        lsum = 0

        for i in nums[:l//2][::-1]:

            lsum += i

            if lsum > s1:

                s1 = lsum

         

        s2 = nums[l//2]

        rsum=0

        for j in nums[l//2:]:

            rsum += j

            if rsum > s2:

                s2 = rsum

        sum = s1+s2

        if sum<left:

            sum = left

        if sum<right:

            sum = right

        return sum

结果：Accepted