寻找最大的子串（分治法）

O(nlgn)

#!/usr/bin/python'''file name: maxsum.py   --P71   --find maximum subarray sum, divide-and-conquer O(nlng)   --author: zevolo, 2012.05.11'''def find_max_across_mid(list, low, high, mid):    ret = list[mid]    right = mid    sum = ret     for i in range(mid + 1, high + 1):        sum = sum + list[i]        if sum > ret:            ret = sum             right = i     left = mid    for i in range(mid - 1, low - 1, -1):        sum = sum + list[i]        if sum > ret:            ret = sum            left = i    return left, right, ret            def find_max_sum(list, low, high):    if high == low:        return (low, high, list[low])        mid = (low + high) / 2    (l_low, l_high, l_sum) = find_max_sum(list, low, mid)    (r_low, r_high, r_sum) = find_max_sum(list, mid + 1, high)    (m_low, m_high, m_sum) = find_max_across_mid(list, low, high, mid)    sum = max(l_sum, r_sum)    sum = max(sum, m_sum)    s_low = r_low    s_high = r_high    if sum == l_sum:        s_low = l_low        s_high = l_high    elif sum == m_sum:        s_low = m_low        s_high = m_high            return (s_low, s_high, sum)if __name__ == '__main__':    l = [1, -2, 3, 10, -4, 7, 2, -5]    print l    (low, high, sum) = find_max_sum(l, 0, len(l) - 1)    out = l[low:high+1]    print (out, sum)