最多两次交易的情况下求股票最大利润

def dp(prices, lentgh):
    res = 0
    dp = [[0 for i in range(lentgh)] for j in range(lentgh)]

    for i in range(lentgh-1):
        for j in range(i+1, lentgh):
            dp[i][j] = prices[j]-prices[i]
    #print dp
    r_max = 0
    ma2 =0
    start = 0
    end=0
    for i in range(lentgh):
        for j in range(i+1, lentgh):
            if dp[i][j] > r_max:
                r_max = dp[i][j]
                start = i
                end = j
    for i in range(start):
        for j in range(i+1,start):
            if dp[i][j] > ma2:
                ma2 = dp[i][j]
    if end < lentgh-1:
        for i in range(end+1, lentgh):
            for j in range(i+1, lentgh):
                if dp[i][j] > ma2:
                    ma2 = dp[i][j]
    res = r_max+ma2
    return res