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# 排序
## 常考排序
### 快速排序
```c++
template
<
typename
T>
static
void
QuickSort
(T arr[],
int
len) {
quickSort
(arr,
0
, len -
1
);
}
template
<
typename
T>
static
void
quickSort
(T arr[],
int
begin,
int
end) {
if
(begin >= end) {
return
;
}
auto
pivot =
partition
(arr, begin, end);
quickSort
(arr, begin, pivot -
1
);
quickSort
(arr, pivot +
1
, end);
}
template
<
typename
T>
static
int
partition
(T arr[],
int
begin,
int
end) {
auto
base = arr[end];
auto
lessInsert = begin;
for
(
int
i = begin; i < end; ++i) {
if
(arr[i] < base) {
swap
(arr[lessInsert++], arr[i]);
}
}
swap
(arr[lessInsert], arr[end]);
return
lessInsert;
}
```
### 归并排序
```c++
template
<
typename
T>
static
void
MergeSort
(T arr[],
int
len) {
auto
tmp =
new
T[len];
mergeSort
(arr,
0
, len -
1
, tmp);
delete[]
tmp;
}
template
<
typename
T>
static
void
mergeSort
(T arr[],
int
begin,
int
end, T tmp[]) {
if
(begin +
1
>= end) {
return
;
}
auto
mid = begin + (end - begin) /
2
;
auto
begin1 = begin;
auto
end1 = mid;
auto
begin2 = mid +
1
;
auto
end2 = end;
mergeSort
(arr, begin1, end1, tmp);
mergeSort
(arr, begin2, end2, tmp);
//
merge two parts
auto
index = begin;
while
(begin1 <= end1 && begin2 <= end2) {
tmp[index++] = arr[begin1] < arr[begin2] ? arr[begin1++] : arr[begin2++];
}
while
(begin1 <= end1) {
tmp[index++] = arr[begin1++];
}
while
(begin2 <= end2) {
tmp[index++] = arr[begin2++];
}
for
(
int
i = begin; i <= end; ++i) {
arr[i] = tmp[i];
}
}
```
### 堆排序
用数组表示的完美二叉树 complete binary tree
> 完美二叉树
VS
其他二叉树
[动画展示](https:
//
www.bilibili.com/video/av18980178/)
核心代码
```c++
template
<
typename
T>
static
void
HeapSort
(T arr[],
int
len) {
//
right leaf of a root = 2(n + 1) since n is start from 0
auto
lastRoot = len /
2
-
1
;
//
先自下而上构建堆
for
(
int
i = lastRoot; i >=
0
; --i) {
makeHeap
(arr, i, len);
}
for
(
int
j = len -
1
; j >=
0
; --j) {
//
最大值放到后面
swap
(arr[
0
], arr[j]);
//
底部都还是有序的，只动了根节点，自上而下更新堆
makeHeap
(arr,
0
, --len);
}
}
template
<
typename
T>
static
void
makeHeap
(T arr[],
int
root,
int
len) {
auto
left = root *
2
+
1
;
auto
right = (root +
1
) *
2
;
auto
largest = root;
if
(left < len && arr[left] > arr[largest]) {
largest = left;
}
if
(right < len && arr[right] > arr[largest]) {
largest = right;
}
if
(largest == root) {
return
;
}
swap
(arr[largest], arr[root]);
makeHeap
(arr, largest, len);
}
```
## 参考
[十大经典排序](https:
//
www.cnblogs.com/onepixel/p/7674659.html)
[二叉堆](https:
//
labuladong.gitbook.io/algo/shu-ju-jie-gou-xi-lie/er-cha-dui-xiang-jie-shi-xian-you-xian-ji-dui-lie)
## 练习
- [ ] 手写快排、归并、堆排序
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