字符串反转

• 翻转字符串（python）
• 翻转中间由各种符号隔开的字符串
• 单词反转（只允许O（1）空间）
• 旋转字符串，n位字符串，循环右移m位，要求时间线性，空间n
• 旋转10次字符串
• 把一个字符串的小写字母放到前面，大写放到后面，保持原有的顺序。（python）

一、反转字符串

思路1：采用中间变量替换

思路2：采用异或的形式【注意：python中字符串不能直接异或，需要采用ord和chr来转化】

#反转字符串
def reverseStr(s):
    if not s:
        return s
    ch = list(s)
    i , j = 0 , len(s)-1
    while i < j:
        ch[i] = chr(ord(ch[i]) ^ ord(ch[j]))
        ch[j] = chr(ord(ch[i]) ^ ord(ch[j]))
        ch[i] = chr(ord(ch[i]) ^ ord(ch[j]))
        # ch[i] = ch[i] ^ ch[j]  【错误的形式】
        # ch[j] = ch[i] ^ ch[j]
        # ch[i] = ch[i] ^ ch[j]
        i += 1
        j -= 1
    return "".join(ch)

s = 'abcdef'
print(reverseStr(s))

二、反转单词

思路：整体全部翻转，再每个单词翻转

代码：

#反转字符串
def reverseStr(s):
    if not s:
        return s
    ch = list(s)
    i , j = 0 , len(s)-1
    while i < j:
        ch[i] = chr(ord(ch[i]) ^ ord(ch[j]))
        ch[j] = chr(ord(ch[i]) ^ ord(ch[j]))
        ch[i] = chr(ord(ch[i]) ^ ord(ch[j]))
        i += 1
        j -= 1
    return "".join(ch)

#s = 'abcdef'
#print(reverseStr(s))

#将句子反转，单词不反转。
def reverseWord(s):
    if not s:
        return s
    rs = reverseStr(s)
    arr = rs.split(' ')
    res = []
    for a in arr:
        res.append(reverseStr(a))
    return ' '.join(res)

s = 'I love dogs'
print(reverseWord(s))

代码2：

def rotateWord(words):
    if words == None or len(words) == 0:
        return
    words = list(words[::-1])
    l , r = -1 , -1
    for i in range(len(words)):
        if words[i] != ' ':
            l = i if (i == 0 or words[i-1] ==' ') else l
            r = i if (i == len(words)-1 or words[i+1] == ' ') else r
        if l != -1 and r != -1:
            # reverse(words,l,r)
            words[l:r+1] = words[l:r+1][::-1]
            l , r = -1 , -1
    return ''.join(words)
    
# def reverse(words,l,r):
#     tmp = ''
#     while l<r:
#         tmp = words[l]
#         words[l] = words[r]
#         words[r] = tmp
#         l += 1
#         r -= 1
words = 'dogs love pigs'
rotateWord(words)

 

题目：循环右移字符串：

思路一：三步反转法 ：(X^TY^T)^T=YX

代码：

##将字符串的size大小的左半区移到右边，如size=3，“ABCDE”变成“DEABC”
###三步反转法
def reverseStr(s):
    if not s:
        return s
    ch = list(s)
    i , j = 0, len(s)-1
    while i < j:
        ch[i] = chr(ord(ch[i])^ord(ch[j]))
        ch[j] = chr(ord(ch[i])^ord(ch[j]))
        ch[i] = chr(ord(ch[i])^ord(ch[j]))
        i += 1
        j -= 1
    return "".join(ch)
def reroteStr(s,size):
    if not s:
        return s
    if size > len(s):
        return s
    s = reverseStr(s)
    n = len(s) - size
    left = reverseStr(s[:n])
    right = reverseStr(s[size-1:])
    return left + right

s = 'ABCDE'
size = 3
print(reroteStr(s,size))

题目：旋转10次字符串

def relote(s):
    for i in range(10):
        s = s[1:] + s[0]
    return s

题目：把一个字符串的小写字母放到前面，大写放到后面，保持原有的顺序。（python）

思路：首尾指针查找大小写字母，替换

代码：

def sortStr(ss):
    if len(ss)<=1:
        return ss
    s = list(ss)
    i = 0
    j = len(s)-1
    while i != j:
        if s[j].islower():
            s[i] , s[j] = s[j] , s[i]
            i += 1
        elif s[j].isupper():
            j -= 1
    return ''.join(s)
s = 'aHjDFksn'
print(sortStr(s))