《算法导论》学习总结 — 第八章(2) 计数排序 && 基数排序 && 桶排序


《算法导论》学习总结 — 8.第八章(2) 计数排序 && 基数排序 && 桶排序

1)稳定的:如果存在多个具有相同排序码的记录,经过排序后,这些记录的相对次序仍然保持不变,则这种排序算法称为稳定的。
插入排序、冒泡排序、归并排序、分配排序(桶式、基数)都是稳定的排序算法。
2)不稳定的:否则称为不稳定的。
直接选择排序、堆排序、shell排序、快速排序都是不稳定的排序算法。
一个各种排序算法演示网站:http://www.sorting-algorithms.com/

计数排序
元素必须是大于等于0,同时不能太大,否则数组res会暴栈
特点是:非常稳定
作用:经常用作基数排序算法的一个子过程
void CountingSort(int kp[], int res[], int size) {
    memset(Hash, 0, sizeof(*Hash) * size);
    for (int i = 1; i <= length; i++) {
        Hash[kp[i]]++;
    }
    //此时Hash[]包含等于i的元素个数
    for (int i = 1; i <= size; i++) {
        Hash[i] += Hash[i - 1];
    }
    //此时Hash[]包含小于等于i的元素个数
    for (int i = length; i >= 1; i--) {
        res[Hash[kp[i]]] = kp[i];
        Hash[kp[i]]--;
    }
}
基数排序
http://massivealgorithms.blogspot.com/2014/07/radix-sort-geeksforgeeks.html
http://massivealgorithms.blogspot.com/2014/08/sort-n-numbers-in-range-from-0-to-n2-1.html
首先要搞懂MSD(Most significant digital, 最高位优先级)和LSD(Least significant digital, 最低位优先级)。例如: 纸牌排序问题,要求结果符合面值,花色从小到大排序
K0 : 花色 (高优先级) ;
K1 : 面值 (低优先级);
MSD:先按k0排序,得到四堆牌,每堆包含属于同一花色的所有纸牌然后分别对每一堆按照面值排序,然后四堆牌叠到一起就是最后结果
LSD:先按k1排序,分成13堆,每堆4种花色,面值相同,然后叠为一堆,然后按k0排序,分为4堆,然后叠为一堆,就是结果。
这期间的排序使用的是计数排序……
可以发现MSD每次都是单独子堆分别排序,而LSD每次都是整堆排序,所以LSD比MSD简单,花费开销要小,所以基数排序通常使用LSD,此处的基数,代表用基数r可以分解为多少个个位数。
r = 10, 基数为10, 按10进制分解结果
r = 2, 基数为2, 按2进制分解结果
排序过程即:先按个位数排序,然后按十位数排序,然后百位数排序,类推……
严蔚敏版《数据结构》中基数排序演示

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