merge_sort and quick_sort in python

python/12_sorts/merge_sort.py

@@ -0,0 +1,48 @@
+"""
+    Author: Wenru
+"""
+
+from typing import List
+
+def merge_sort(a: List[int]):
+    _merge_sort_between(a, 0, len(a)-1)
+
+def _merge_sort_between(a: List[int], low: int, high: int):
+    # The indices are inclusive for both low and high.
+    if low >= high: return
+    mid = low + (high - low)//2
+    _merge_sort_between(a, low, mid)
+    _merge_sort_between(a, mid+1, high)
+
+    _merge(a, low, mid, high)
+
+def _merge(a: List[int], low: int, mid: int, high: int):
+    # a[low:mid], a[mid+1, high] are sorted.
+    i, j = low, mid+1
+    tmp = []
+    while i <= mid and j <= high:
+        if a[i] <= a[j]:
+            tmp.append(a[i])
+            i += 1
+        else:
+            tmp.append(a[j])
+            j += 1
+    start = i if i <= mid else j
+    end = mid if i <= mid else high
+    tmp.extend(a[start:end+1])
+    a[low:high+1] = tmp
+
+
+if __name__ == "__main__":
+    a1 = [3, 5, 6, 7, 8]
+    a2 = [2, 2, 2, 2]
+    a3 = [4, 3, 2, 1]
+    a4 = [5, -1, 9, 3, 7, 8, 3, -2, 9]
+    merge_sort(a1)
+    print(a1)
+    merge_sort(a2)
+    print(a2)
+    merge_sort(a3)
+    print(a3)
+    merge_sort(a4)
+    print(a4)

python/12_sorts/quick_sort.py

@@ -0,0 +1,43 @@
+"""
+    Author: Wenru
+"""
+
+from typing import List
+import random
+
+def quick_sort(a: List[int]):
+    _quick_sort_between(a, 0, len(a)-1)
+
+def _quick_sort_between(a: List[int], low: int, high: int):
+    if low >= high: return
+    # get a random position as the pivot
+    k = random.randint(low, high)
+    a[low], a[k] = a[k], a[low]
+
+    m = _partition(a, low, high)   # a[m] is in final position
+    _quick_sort_between(a, low, m-1)
+    _quick_sort_between(a, m+1, high)
+
+def _partition(a: List[int], low: int, high: int):
+    pivot, j = a[low], low
+    for i in range(low+1, high+1):
+        if a[i] <= pivot:
+            j += 1
+            a[j], a[i] = a[i], a[j]  # swap
+    a[low], a[j] = a[j], a[low]
+    return j
+
+
+if __name__ == "__main__":
+    a1 = [3, 5, 6, 7, 8]
+    a2 = [2, 2, 2, 2]
+    a3 = [4, 3, 2, 1]
+    a4 = [5, -1, 9, 3, 7, 8, 3, -2, 9]
+    quick_sort(a1)
+    print(a1)
+    quick_sort(a2)
+    print(a2)
+    quick_sort(a3)
+    print(a3)
+    quick_sort(a4)
+    print(a4)