[英]Whats wrong with quicksort3
我的三向分區快速排序有什么問題? 當輸入的數據少於100000時,它將正常工作。 當輸入數據= 100000時,它的工作時間約為9秒。 我使用Dijkstra 3路分區。 如果輸入數據由大量相同的元素組成,則一切正常,而輸入數據隨機工作太慢。
static void randomizedQuickSort(int[] a, int l, int r) {
if (l >= r) {
return;
}
int[] m = Partition3(a, l, r);
randomizedQuickSort(a, l, m[0] - 1);
randomizedQuickSort(a, m[1] + 1, r);
}
private static int[] Partition3(int[] nums, int l, int r) {
Random random = new Random();
int k = random.nextInt(r - l + 1) + l;
int mid = nums[k];
int m1 = 0;
int i = 0;
int m2 = r;
while (m1 <= m2) {
if (nums[m1] < mid) {
swap(nums, i, m1);
i++;
m1++;
} else if (nums[m1] > mid) {
swap(nums, m1, m2);
m2--;
} else {
m1++;
}
}
return new int[]{i, m2};
}
您做錯了什么,因為交換數為O(n ^ 2)如果計算swap
的次數,您會得到類似的信息(第一個數字是元素數)
10000: Took 0.078000 seconds, and 33432534 swaps
20000: Took 0.291000 seconds, and 166934755 swaps
40000: Took 1.102000 seconds, and 702291723 swaps
80000: Took 4.482000 seconds, and 2837543629 swaps
160000: Took 17.590000 seconds, and 11373050608 swaps
問題是線路
int m1 = 0;
int i = 0;
在每種排序中,您都從數組的開頭進行排序。
int m1 = l; // sort from the start of the section.
int i = l;
完整版本是...。
public static void main(String[] args) {
for (int t = 100_000; t <= 100_000_000; t *= 10) {
int[] nums = new int[t];
for (int i = 0; i < nums.length; i++) {
nums[i] = random.nextInt();
}
long start = System.currentTimeMillis();
swaps = 0;
randomizedQuickSort(nums, 0, nums.length - 1);
long time = System.currentTimeMillis() - start;
for (int i=0;i<nums.length-1;i++)
if (nums[0] > nums[1])
throw new AssertionError();
System.out.printf("%d: Took %f seconds, and %d swaps%n", t, time / 1e3, swaps);
}
}
static void randomizedQuickSort(int[] a, int l, int r) {
if (l >= r) {
return;
}
long m = Partition3(a, l, r);
int m0 = (int) (m >> 32);
int m1 = (int) m;
randomizedQuickSort(a, l, m0 - 1);
randomizedQuickSort(a, m1 + 1, r);
}
static final Random random = new Random();
static long swaps = 0;
private static long Partition3(int[] nums, int l, int r) {
int k = random.nextInt(r - l + 1) + l;
int mid = nums[k];
int m1 = l;
int i = l;
int m2 = r;
while (m1 <= m2) {
if (nums[m1] < mid) {
swap(nums, i, m1);
i++;
m1++;
} else if (nums[m1] > mid) {
swap(nums, m1, m2);
m2--;
} else {
m1++;
}
}
return ((long) i << 32) | m2;
}
private static void swap(int[] nums, int i, int j) {
int t = nums[i];
nums[i] = nums[j];
nums[j] = t;
swaps++;
}
版畫
100000: Took 0.018000 seconds, and 2032183 swaps
1000000: Took 0.168000 seconds, and 24872604 swaps
10000000: Took 1.709000 seconds, and 287681791 swaps
100000000: Took 19.015000 seconds, and 3353327832 swaps
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