Quicksort optimization

Order check & smartly set pivot

template/quicksort.cpp

@@ -1,54 +1,102 @@
 //快速排序
-#include<cstdio>
-#include<iostream>
+#include <cstdio>
+#include <iostream>
 using namespace std;
-void quicksort(int num[],int l,int r);
-void swap(int* a,int* b);
+void quicksort(int num[], const int &l, const int &r);
+template <typename T>
+void swap(T *a, T *b);
+int getPivot(int num[], const int &l, const int &r);
+bool checkOrder(int num[], const int &l, const int &r);
 int main()
 {
-    int num[100000],n,a;
-    scanf("%d",&n);
-    for(a=0;a<n;a++)
+    int num[100000], n, a;
+    scanf("%d", &n);
+    for (a = 0; a < n; a++)
     {
-        scanf("%d",&num[a]);
+        scanf("%d", &num[a]);
     }
-    quicksort(num,0,n-1);
-    for(a=0;a<n;a++)
+    quicksort(num, 0, n - 1);
+    for (a = 0; a < n; a++)
     {
-        printf("%d ",num[a]);
+        printf("%d ", num[a]);
     }
     return 0;
 }
-void quicksort(int num[],int l,int r)
+void quicksort(int num[], const int &l, const int &r) //数组，左边界，右边界
 {
-    if(l>=r)
+    if (l >= r) //区间不可再排序
     {
         return;
     }
-    int i,j,pivot=*(num+l);
-    i=l;
-    j=r;
-    while(i<j)
+    if (checkOrder(num, l, r)) //这个区间是有序的
     {
-        while(*(num+j)>=pivot&&i<j)
+        printf("Ordered between %d and %d, returning...");
+        return;
+    }
+    int i, j, pivotpos = getPivot(num, l, r);
+    int pivot = num[pivotpos];
+    printf("L=%d R=%d\n", l, r);
+    printf("Pivot selected: %d\n", pivot);
+    swap(num + l, num + pivotpos);
+    i = l;
+    j = r;
+    while (i < j)
+    {
+        while (num[j] >= pivot && i < j) //移到右边第一个比pivot小的数
         {
             j--;
         }
-        while(*(num+i)<=pivot&&i<j)
+        while (num[i] <= pivot && i < j) //移到左边第一个比pivot大的数
         {
             i++;
         }
-        if(i<j)swap(num+i,num+j);
+        if (i < j)
+        {
+            swap(num + i, num + j);
+            printf("Swapped %d and %d\n", num[j], num[i]);
+        }
     }
-    swap(num+l,num+i);
-    quicksort(num,l,i-1);
-    quicksort(num,i+1,r);
+    swap(num + l, num + i);
+    printf("Swapped %d and %d\n", num[i], num[l]);
+    quicksort(num, l, i - 1);
+    quicksort(num, i + 1, r);
     return;
 }
-void swap(int* a,int* b)
+template <typename T>
+void swap(T *a, T *b)
 {
-    int temp=*a;
-    *a=*b;
-    *b=temp;
+    T temp = *a;
+    *a = *b;
+    *b = temp;
     return;
+}
+int getPivot(int num[], const int &l, const int &r)
+{
+    pair<int, int> lVal(num[l], l);
+    pair<int, int> rVal(num[r], r);
+    pair<int, int> midVal(num[(l + r) / 2], (l + r) / 2);
+    if (rVal > midVal)
+    {
+        swap(&rVal, &midVal);
+    }
+    if (midVal > lVal)
+    {
+        swap(&lVal, &midVal);
+    }
+    if (rVal > midVal)
+    {
+        swap(&rVal, &midVal);
+    }
+    return midVal.second;
+}
+bool checkOrder(int num[], const int &l, const int &r)
+{
+    for (int i = l; i < r; i++)
+    {
+        if (num[i] > num[i + 1])
+        {
+            return false;
+        }
+    }
+    return true;
 }