Question

有人可以解释为什么下面的代码不适用于所有情况吗？例如，在无法正常工作的情况下-

int a[]={ -120, -202, -293, -60, -261, -67, 10, 82, -334, -393, -428, -182, -138, -167, -465, -347, -39, -51, -61, -491, -216, -36, -281, -361, -271, -368, -122, -114, -53, -488, -327, -182, -221, -381, -431, -161, -59, -494, -406, -298, -268, -425, -88, -320, -371, -5, 36, 89, -194, -140, -278, -65, -38, -144, -407, -235, -426, -219, 62, -299, 1, -454, -247, -146, 24, 2, -59, -389, -77, -19, -311, 18, -442, -186, -334, 41, -84, 21, -100, 65, -491, 94, -346, -412, -371, 89, -56, -365, -249, -454, -226, -473, 91, -412, -30, -248, -36, -95, -395, -74, -432, 47, -259, -474, -409, -429, -215, -102, -63, 80, 65, 63, -452, -462, -449, 87, -319, -156, -82, 30, -102, 68, -472, -463, -212, -267, -302, -471, -245, -165, 43, -288, -379, -243, 35, -288, 62, 23, -444, -91, -24, -110, -28, -305, -81, -169, -348, -184, 79, -262, 13, -459, -345, 70, -24, -343, -308, -123, -310, -239, 83, -127, -482, -179, -11, -60, 35, -107, -389, -427, -210, -238, -184, 90, -211, -250, -147, -272, 43, -99, 87, -267, -270, -432, -272, -26, -327, -409, -353, -475, -210, -14, -145, -164, -300, -327, -138, -408, -421, -26, -375, -263, 7, -201, -22, -402, -241, 67, -334, -452, -367, -284, -95, -122, -444, -456, -152, 25, 21, 61, -320, -87, 98, 16, -124, -299, -415, -273, -200, -146, -437, -457, 75, 84, -233, -54, -292, -319, -99, -28, -97, -435, -479, -255, -234, -447, -157, 82, -450, 86, -478, -58, 9, -500, -87, 29, -286, -378, -466, 88, -366, -425, -38, -134, -184, 32, -13, -263, -371, -246, 33, -41, -192, -14, -311, -478, -374, -186, -353, -334, -265, -169, -418, 63, 77, 77, -197, -211, -276, -190, -68, -184, -185, -235, -31, -465, -297, -277, -456, -181, -219, -329, 40, -341, -476, 28, -313, -78, -165, -310, -496, -450, -318, -483, -22, -84, 83, -185, -140, -62, -114, -141, -189, -395, -63, -359, 26, -318, 86, -449, -419, -2, 81, -326, -339, -56, -123, 10, -463, 41, -458, -409, -314, -125, -495, -256, -388, 75, 40, -37, -449, -485, -487, -376, -262, 57, -321, -364, -246, -330, -36, -473, -482, -94, -63, -414, -159, -200, -13, -405, -268, -455, -293, -298, -416, -222, -207, -473, -377, -167, 56, -488, -447, -206, -215, -176, 76, -304, -163, -28, -210, -18, -484, 45, 10, 79, -441, -197, -16, -145, -422, -124, 79, -464, -60, -214, -457, -400, -36, 47, 8, -151, -489, -327, 85, -297, -395, -258, -31, -56, -500, -61, -18, -474, -426, -162, -79, 25, -361, -88, -241, -225, -367, -440, -200, 38, -248, -429, -284, -23, 19, -220, -105, -81, -269, -488, -204, -28, -138, 39, -389, 40, -263, -297, -400, -158, -310, -270, -107, -336, -164, 36, 11, -192, -359, -136, -230, -410, -66, 67, -396, -146, -158, -264, -13, -15, -425, 58, -25, -241, 85, -82, -49, -150, -37, -493, -284, -107, 93, -183, -60, -261, -310, -380
    };

我在看完坎丹算法后就写了代码。

int maxSubArray(int a[],int n) 
{
    int global=-99999,local=0;
    for(int i=0;i<n;i++)
    {
        if(a[i]>global)
        {
            global=max(a[i],local+a[i]);
            local=global;
            continue;
        }
        local+=a[i];
        if(local>global)
        {global=local;}
    }
    return global;
}

我的输出为114，要求的输出为217

Answer 1

您使问题有点复杂。

int maxSubArray(int a[], int n)
{
    int global = -99999, local = 0; 
    //if you are trying to set the biggest negative number, use INT_MIN.
    for (int i = 0; i < n; i++)
    {
        if (a[i] > global) 
            //Why do you check a[i] is bigger than global? if global becomes a huge number, the code inside of this if
            //never will execute
        {
            global = max(a[i], local + a[i]);
            local = global;
            continue;
        }
        local += a[i]; //This is good

        if (local < a[i]) 
            //Another confusing point, you are trying to get the sum of continuous array. 
            //If a[i] is a huge number, local will be modified no matter there were some positive numbers before a[i]
            //Which easily can cause inaccurate results
        {
            local = a[i];
        }

        if (local > global) // This is not wrong but it could be better
        {
            global = local;
        }
    }
    return global;
}

总的来说，我认为您可能需要在O（n ^ 2）的天真的版本上使用更多的实践，该版本首先使用嵌套循环。

下面的代码是经过修改的版本

int maxSubArray(int a[], int n)
{
    int sum = 0; 
    int largest = INT_MIN; //reason why I set it as int_min is there are cases that an array contains only negative numbers
    for (int i = 0; i < n; i++) //looping  
    {
        sum += a[i]; // add a[i] to the sum variable
        largest = max(largest, sum); 
        // compare the current sum to the current largest number
        // An if statement is not required because "largest" variable always is set to be bigger number
        if (sum < 0) 
        // if the sum is negative, resets to 0
        // Little bit confusing but it works because when sum becomes below 0, it is definitively not the maximum sum of array
        {
            sum = 0; 
        }
    }
    return largest; // return the largest number
}

Answer 2

您需要添加另一个if条件，以检查local是否小于a [i]并将其更新为a [i]。它将适用于大于-99999的值。您可以将global设置为较小的值，以扩大范围。

int maxSubArray(int a[],int n) 
{
    int global=-99999,local=0;
    for(int i=0;i<n;i++)
    {
        if(a[i]>global)
        {
            global=max(a[i],local+a[i]);
            local=global;
            continue;
        }
        local+=a[i];
        if(local<a[i])
        {
            local=a[i];
        }
        if(local>global)
        {global=local;}
}
return global;
}

最大连续和

2 个答案: