算法的运行时间并不以秒为单位。
算法的运行时间是其从增速的角度衡量的。
算法的运行时间用大O表示法表示
1.二分查找输入是一个有序列表。
def binary_search(list1, item):
"""
二分查找的速度比简单查找快很多。
O(log n)比O(n)快。需要搜索的元素越多，前者就比后者快得越多。
"""
start = 0
end = len(list1) - 1
while start <= end:
middle = int((start+ end) / 2)
guess = list1[middle]
if guess == item:
return middle
elif guess > item:
end = middle - 1
else:
start = middle+1
return None
2.选择排序
def findSmallest(arr):
smallest = arr[0]
small_index = 0
for i in range(1, len(arr)):
if arr[i] < smallest:
smallest = arr[i]
small_index = i
return small_index
def selectionSort(arr):
out = []
while arr:
out.append(arr.pop(findSmallest(arr)))
return out
3.快速排序
def quicksort(array):
if len(array) < 2:
return array
else:
pivot = array[0]
less = [i for i in array[1:] if i < pivot]
greater = [i for i in array[1:] if i > pivot]
return quicksort(less) + [pivot] + quicksort(greater)
4.广度优先搜索
def search():
graph = {}
graph["you"] = ["alice", "bob", "claire"]
graph["bob"] = ["anuj", "peggy"]
graph["alice"] = ["peggy"]
graph["claire"] = ["thom", "jonny"]
graph["anuj"] = []
graph["peggy"] = []
graph["thom"] = []
graph["jonny"] = []
from collections import deque
search_queue = deque()
search_queue += graph["you"]
def person_is_seller(name):
return name[-1] == 'm'
while search_queue:
person = search_queue.popleft()
if person_is_seller(person):
print(person+"is a mango seller")
return True
else:
search_queue += graph[person]
return False
5.狄克斯特拉算法
graph = {}
graph["start"] = {}
graph["start"]["a"] = 6
graph["start"]["b"] = 2
graph["a"] = {}
graph["a"]["end"] = 1
graph["b"] = {}
graph["b"]["a"] = 3
graph["b"]["end"] = 5
graph["end"] = {}
infinity = float("inf")
costs = {}
costs["a"] = 6
costs["b"] = 2
costs["end"] = infinity
parents = {}
parents["a"] = "start"
parents["b"] = "start"
parents["end"] = None
processed= []
def find_lowest_cost_node(costs):
lowest_cost = float("inf")
lowest_cost_node = None
for node in costs:
cost = costs[node]
if cost < lowest_cost and node not in processed:
lowest_cost = cost
lowest_cost_node = node
return lowest_cost_node
node = find_lowest_cost_node(costs)
while node is not None:
nb = graph[node]
for n in nb.keys():
new = costs[node]+nb[n]
if costs[n] > new:
costs[n] = new
parents[n] = node
processed.append(node)
node = find_lowest_cost_node(costs)