Poison

剑指 Offer 40. 最小的k个数

Priority Queue
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
class Solution {
public int[] getLeastNumbers(int[] arr, int k) {
if (k == 0) {
return new int[0];
}

PriorityQueue<Integer> maxHeap = new PriorityQueue<>((o1, o2) -> o2 - o1);

for (int i = 0; i < k; i++) {
maxHeap.offer(arr[i]);
}

for (int i = k; i < arr.length; i++) {
if (arr[i] < maxHeap.peek()) {
maxHeap.offer(arr[i]);
maxHeap.poll();
}
}

int[] res = new int[k];
for (int i = 0; i < res.length; i++) {
res[i] = maxHeap.poll();
}
return res;
}
}
QuickSort
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
class Solution {
public int[] getLeastNumbers(int[] arr, int k) {
if (k == 0) {
return new int[0];
}

int targetIndex = k - 1;
quickSort(arr, 0, arr.length - 1, targetIndex);
return Arrays.copyOf(arr, k);
}

private void quickSort(int[] arr, int i, int j, int targetIndex) {
if (i >= j) {
return;
}

int pivotIndex = partition(arr, i, j);
if (pivotIndex == targetIndex) {
return;
}
quickSort(arr, i, pivotIndex - 1, targetIndex);
quickSort(arr, pivotIndex + 1, j, targetIndex);
}

private int partition(int[] arr, int i, int j) {
int pivotIndex = i + ThreadLocalRandom.current().nextInt(j - i + 1); // i + [0, j - i] -> [i, j]
int pivotValue = arr[pivotIndex];
swap(arr, pivotIndex, i);

int index = i;
for (int k = i + 1; k <= j; k++) {
if (arr[k] <= pivotValue) {
swap(arr, k, ++index);
}
}

swap(arr, i, index);
return index;
}

private void swap(int[] nums, int i, int j) {
int tmp = nums[i];
nums[i] = nums[j];
nums[j] = tmp;
}
}

该解法效率不高的原因为为 pivotIndex 左右侧的数组都进行了排序,可以优化为仅对 targetIndex 一侧的数组进行排序。

QuickSort
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
class Solution {
public int[] getLeastNumbers(int[] arr, int k) {
if (k == 0) {
return new int[0];
}

int targetIndex = k - 1;
quickSort(arr, 0, arr.length - 1, targetIndex);
return Arrays.copyOf(arr, k);
}

private void quickSort(int[] arr, int i, int j, int targetIndex) {
if (i >= j) {
return;
}

int pivotIndex = partition(arr, i, j);
if (pivotIndex < targetIndex) {
quickSort(arr, pivotIndex + 1, j, targetIndex);
} else if (pivotIndex > targetIndex) {
quickSort(arr, i, pivotIndex - 1, targetIndex);
}
}

private int partition(int[] arr, int i, int j) {
int pivotIndex = i + ThreadLocalRandom.current().nextInt(j - i + 1); // i + [0, j - i] -> [i, j]
int pivotValue = arr[pivotIndex];
swap(arr, pivotIndex, i);

int index = i;
for (int k = i + 1; k <= j; k++) {
if (arr[k] <= pivotValue) {
swap(arr, k, ++index);
}
}

swap(arr, i, index);
return index;
}

private void swap(int[] nums, int i, int j) {
int tmp = nums[i];
nums[i] = nums[j];
nums[j] = tmp;
}
}
QuickSort
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
class Solution {
public int[] getLeastNumbers(int[] arr, int k) {
if (k == 0) {
return new int[0];
}

int targetIndex = k - 1;
return quickSort(arr, 0, arr.length - 1, targetIndex);
}

private int[] quickSort(int[] arr, int i, int j, int targetIndex) {
int pivotIndex = partition(arr, i, j);
if (pivotIndex < targetIndex) {
return quickSort(arr, pivotIndex + 1, j, targetIndex);
} else if (pivotIndex > targetIndex) {
return quickSort(arr, i, pivotIndex - 1, targetIndex);
} else {
return Arrays.copyOf(arr, targetIndex + 1);
}
}

private int partition(int[] arr, int i, int j) {
int pivotIndex = i + ThreadLocalRandom.current().nextInt(j - i + 1); // i + [0, j - i] -> [i, j]
int pivotValue = arr[pivotIndex];
swap(arr, pivotIndex, i);

int index = i;
for (int k = i + 1; k <= j; k++) {
if (arr[k] <= pivotValue) {
swap(arr, k, ++index);
}
}

swap(arr, i, index);
return index;
}

private void swap(int[] nums, int i, int j) {
int tmp = nums[i];
nums[i] = nums[j];
nums[j] = tmp;
}
}
QuickSort + Two Pointers
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
class Solution {
public int[] getLeastNumbers(int[] arr, int k) {
if (k == 0) {
return new int[0];
}

int targetIndex = k - 1;
return quickSort(arr, 0, arr.length - 1, targetIndex);
}

private int[] quickSort(int[] arr, int i, int j, int targetIndex) {
int pivotIndex = partition(arr, i, j);
if (pivotIndex < targetIndex) {
return quickSort(arr, pivotIndex + 1, j, targetIndex);
} else if (pivotIndex > targetIndex) {
return quickSort(arr, i, pivotIndex - 1, targetIndex);
} else {
return Arrays.copyOf(arr, targetIndex + 1);
}
}

private int partition(int[] arr, int l, int r) {
int i = l, j = r;
while (i < j) {
while (i < j && arr[j] >= arr[l]) {
j--;
}
while (i < j && arr[i] <= arr[l]) {
i++;
}
swap(arr, i, j);
}

swap(arr, i, l);
return i;
}

private void swap(int[] nums, int i, int j) {
int tmp = nums[i];
nums[i] = nums[j];
nums[j] = tmp;
}
}
Reference

剑指 Offer 40. 最小的k个数