Python面试常用算法

查找算法

二分查找

# 实现一个二分查找
# 输入:一个顺序list
# 输出: 待查找的元素的位置
def binarySearch(alist, item):
    first = 0
    last = len(alist) - 1

    while first <= last:
        mid = (first + last)//2
        print(mid)
        if alist[mid] > item:
            last = mid - 1
        elif alist[mid] < item:
            first = mid + 1
        else:
            return mid+1
    return -1

test = [0, 1, 2, 8, 13, 17, 19, 32, 42]
print(binarySearch(test, 3))

排序算法

冒泡排序

def bubbleSort(alist):
    for passnum in range(len(alist)-1, 0, -1):
        for i in range(passnum):
            if alist[i] > alist[i+1]:
                alist[i], alist[i+1] = alist[i+1], alist[i]
    return alist

插入排序

def insertionSort(alist):
    for index in range(1, len(alist)):
        currentvalue = alist[index]
        position = index

        while position > 0 and alist[position-1] > currentvalue:
            alist[position] = alist[position-1]
            position -= 1
        alist[position] = currentvalue

快速排序

def quickSort(alist):
    quickSortHelper(alist, 0, len(alist)-1)

def quickSortHelper(alist, first, last):
    if first < last:
        splitPoint = partition(alist, first, last)

        quickSortHelper(alist, first, splitPoint-1)
        quickSortHelper(alist, splitPoint+1, last)

def partition(alist, first, last):
    pivotvlue = alist[first]

    leftmark = first+1
    rightmark = last
    done = False

    while leftmark > rightmark:
        while alist[leftmark] <= pivotvlue and leftmark <= rightmark:
            leftmark += 1
        while alist[rightmark] >= pivotvlue and rightmark >= leftmark:
            rightmark -= 1
        alist[leftmark], alist[rightmark] = alist[rightmark], alist[leftmark]
    alist[rightmark], alist[first] = alist[first], alist[rightmark]
    return rightmark

选择排序

def selectionSort(alist):
    for i in range(len(alist)-1):
        min = i
        for j in range(i+1, len(alist)):
            if alist[j] < alist[min]:
                min = j
        alist[i], alist[min] = alist[min], alist[i]
    return alist

冒泡排序

class Solution:
    def bubbleSort(nums):
        # 这个循环负责设置冒泡排序进行的次数
        for i in range(len(nums)-1):  
            # ｊ为列表下标  
            for j in range(len(nums)-i-1):  
                if nums[j] > nums[j+1]:
                    nums[j], nums[j+1] = nums[j+1], nums[j]
        return nums

树的四种遍历方式

class node(object):
    def __init__(self,data=None,left=None,right=None):
        self.data=data
        self.left=left
        self.right=right

    #深度
    def depth(tree):
        if tree==None:
            return 0
        left,right=depth(tree.left),depth(tree.right)
        return max(left,right)+1
    #前序遍历   
    def pre_order(tree):
        if tree==None:
            return
        print tree.data
        pre_order(tree.left)
        pre_order(tree.right)
    #中序遍历   
    def mid_order(tree):
        if tree==None:
            return
        mid_order(tree.left)
        print tree.data
        mid_order(tree.right)    
    #后序遍历   
    def post_order(tree):
        if tree==None:
            return
        post_order(tree.left)
        post_order(tree.right)   
        print tree.data

    #层次遍历    
    def level_order(tree):
         if tree==None:
            return 
         q=[]
         q.append(tree)
         while q:
             current=q.pop(0)
             print current.data
             if current.left!=None:
                q.append(current.left)
             if current.right!=None:
                q.append(current.right)