leetcode笔记 动态规划在字符串匹配中的应用
leetcode笔记 动态规划在字符串匹配中的应用
0 参考文献
| 序号 | 标题 |
|---|---|
| 1 | 一招解决4道leetcode hard题，动态规划在字符串匹配问题中的应用 |
| 2 | 10.Regular Expression Matching |
1. [10. Regular Expression Matching]
1.1 题目
Given an input string (s) and a pattern (p), implement regular expression matching with support for '.' and '*'.
'.' Matches any single character.
'*' Matches zero or more of the preceding element.
The matching should cover the entire input string (not partial).
Note:
scould be empty and contains only lowercase lettersa-z.pcould be empty and contains only lowercase lettersa-z, and characters like.or*.
Example 1:
Input:
s = "aa"
p = "a"
Output: false
Explanation: "a" does not match the entire string "aa".
Example 2:
Input:
s = "aa"
p = "a*"
Output: true
Explanation: '*' means zero or more of the precedeng element, 'a'. Therefore, by repeating 'a' once, it becomes "aa".
Example 3:
Input:
s = "ab"
p = ".*"
Output: true
Explanation: ".*" means "zero or more (*) of any character (.)".
Example 4:
Input:
s = "aab"
p = "c*a*b"
Output: true
Explanation: c can be repeated 0 times, a can be repeated 1 time. Therefore it matches "aab".
Example 5:
Input:
s = "mississippi"
p = "mis*is*p*."
Output: false
1.2 思路 && 解题方法
首先建立一个二维数组，数P组的列是代表了字符串S，数组的行代表了字符串P。dp[i] [j] 表示P[0:i] 匹配S[0:j]。因此如果最后P能够匹配S，则dp [len(P)] [len(S)] == True 。注意dp[0] [0] 分别是S是空字符和P是空字符的时候。 这个时候是必定匹配的，因此dp [0] [0] = True。
之后需要做的事情就是依次填满这个矩阵。为此需要初始化dp [ 0 ] [ j ] 和dp [ i ] [ 0 ]既第0行和第0列。
-
对于第0行因为P为空，则除了S是空以外其他的都不匹配。因此 dp [ 0 ] [ j ] = False
-
对于第0列，则需要判断下P是否能匹配S是空串的情况。在S是空串的情况下，之后P是空串或者P是带有 " * "的情况下才能匹配，因此只需要处理这两种情况。
- P是空串的情况下，可以匹配S。因此dp [ 0 ] [ 0 ] = True
- P是" * "的情况下，例如"abc"，因为可以是匹配0个或者多个字符。因此当在这种情况下，*号其实可以消掉前面的字符变成""。因此dp [ i ] [ 0 ] = dp [ i-2 ] [ 0 ] and P [ i -1 ] == " * "。这里为什么不是判断dp [ i -1 ] [ 0 ] 是否为True 而是判断dp [ i-2 ] [ 0 ]呢？是因为dp [ i-1 ] [ 0 ]是表示 P [ 0 : i - 2 ] 能够匹配S [ 0 ]，如果P [ 0 : i -2 ]能够匹配S [ 0 ]，那么当前字符" * " 消掉前一个字符便无法匹配S [ 0 ] (既dp [ i ] [ 0 ] == False)。如下图的示例，当P="a * b * " 是可以匹配""空字符的。那么当i = 2 ，j = 0 的时候，必须有dp [ 0 ] [ 0 ] == True 才能得到 dp [ 2 ] [ 0 ] == True。
到此，dp矩阵的初始化已经好了。这个时候，矩阵中的值如图所示。绿色部分是已经初始化的值。空白的部分是待填充的。
接下来就是填充dp矩阵的剩余部分。对于dp [ i ] [ j ] ( i>1, j>1)会有以下的几种情况：
-
P [ i - 1 ] == " * " ：
对于这种情况，还可以区别2中情况：
-
" * " 抵消前面的字符，既 " * "匹配空字符串：
对于这种情况则和前文所述的方法一样，dp [ i ] [ j ] == dp [ i-2 ] [ j ]
-
" * "匹配前面的字符N次 ：
对于这种情况，则需要在 ( ( P[ i - 1 ] == " . " ) or ( S[ j -1 ] == P [ i -2 ] ) )的情况下 ，dp [ i -1 ] [ j ] == True。这是为什么呢？原因在于如果要匹配0-N次，则代表了P[ 0 - i -2 ] (既dp [ i -1 ] [ XXXX ] ) 能完全匹配S[ 0 : j - 1 ]。
如例子中的 "a." 能匹配 "abb"。
-
-
P [ i - 1 ] == " . " or P [ i - 1 ] == " 一个正常的字符 " :
如果是这种情况见简单的多，既( S [ j - 1 ] == P [ j -1 ] or P [ j -1 ] == " . " ) and dp [ i - 1 ] [ j -1 ] == True 。
1.3 实现
class Solution(object):
def isMatch(self, s, p):
"""
:type s: str
:type p: str
:rtype: bool
"""
# dp[i][j] 代表了p字符串从0-i是否匹配s字符的0-j
row = len(p) + 1
col = len(s) + 1
dp = [ [False for i in range( col ) ] for j in range( row ) ]
dp[0][0] = True # dp[0][0] 代表了p是空串 s是空串
# 当s时空串的情况下，p的不同，匹配的不同情况。为接下去匹配len(s) = 1 ,2 ,3 .... n 做准备
# 当s为空串的时候，只有a*b*这种能匹配。
# 因此dp[0][0] 为空串，所以i-1实际真正指向p的一个字符串的位置
for i in range( 1, row):
dp[i][0] = ( i > 1 ) and p[ i - 1 ] == "*" and dp[ i - 2 ][0]
for i in range( 1, row ) :
for j in range( 1, col ):
if p[ i - 1 ] =="*":
dp[i][j] = dp[ i - 2 ][j] or ( p[ i - 2 ] == s[ j - 1 ] or p[ i - 2 ] == ".") and dp[i][j-1]
else:
dp[i][j] = ( p[ i - 1 ] == "." or p[ i - 1 ] == s[ j - 1 ]) and dp[i-1][j-1]
return dp[row-1][col-1]
2. [44. Wildcard Matching]
2.1 题目
Given an input string (s) and a pattern (p), implement wildcard pattern matching with support for '?' and '*'.
'?' Matches any single character.
'*' Matches any sequence of characters (including the empty sequence).
The matching should cover the entire input string (not partial).
Note:
scould be empty and contains only lowercase lettersa-z.pcould be empty and contains only lowercase lettersa-z, and characters like?or*.
Example 1:
Input:
s = "aa"
p = "a"
Output: false
Explanation: "a" does not match the entire string "aa".
Example 2:
Input:
s = "aa"
p = "*"
Output: true
Explanation: '*' matches any sequence.
Example 3:
Input:
s = "cb"
p = "?a"
Output: false
Explanation: '?' matches 'c', but the second letter is 'a', which does not match 'b'.
Example 4:
Input:
s = "adceb"
p = "*a*b"
Output: true
Explanation: The first '*' matches the empty sequence, while the second '*' matches the substring "dce".
Example 5:
Input:
s = "acdcb"
p = "a*c?b"
Output: false
2.2 思路 && 解题方法
这道题和前面的那道题思路是一样的，也是维护一个二维数组dp 来解题。只不过这里匹配任意字符的符号换成了"?" ,而" * "现在是可以匹配任意序列包括空字符串。 同样的假设当前S= "abc" P="a?b*"，则有如下的dp数组：
同样首先初始化第0行和第0列。
- 对于第0行很好处理，除了0,0位置，其他的地方全部都是False
- 对于第0列，因为S=""因此只有当遇到了" * "的时候，才能匹配。则匹配的条件是dp [ i -1 ] [ 0 ] == True 。
开始填充dp的时候，也是有如下的2种情况：
-
当P [ i-1 ] == " * " :
这种情况下也分2种情况：
- " * "当做空字符串使用：则和前述一样 dp [ i ] [ j ] =( dp [ i -1 ] [ j ] == True )
- " * "当做任意字符串使用 : 则 dp [ i ] [ j ] = ( dp [ i ] [ j - 1 ] == True）。这里解释下我的理解。对于" * "当做任意字符使用的情况下，dp [ i ] [ j - 1 ] == True 表示的是：P [ 0 : i-2 ] 匹配了 S [ 0 : j -2 ]，同时P[ i - 1 ] (当前是 * )当做空子串使用。
-
当P [ i-1 ] == " ? " 或 " 一个正常的字符 "：
则( S [ j - 1 ] == P [ j -1 ] or P [ j -1 ] == " ？" ) and dp [ i - 1 ] [ j -1 ] == True
2.3 实现
#!/bin/python
class Solution(object):
def isMatch(self, s, p):
"""
:type s: str
:type p: str
:rtype: bool
"""
row = len(p) + 1
col = len(s) + 1
dp = [ [False for i in range( 0, col )] for j in range( 0, row ) ]
dp[0][0] = True
for j in range(1, col ):
dp[0][j] = False
for i in range( 1, row):
if p[i-1] == "*":
dp[i][0] = dp[i-1][0]
for i in range( 1, row ):
for j in range( 1, col ):
if p[i-1] == "*":
dp[i][j] = dp[i-1][j] or dp[i][j-1]
else:
dp[i][j] = (s[j-1] == p[i-1] or p[i-1] == "?") and dp[i-1][j-1]
return dp[row-1][col-1]
if __name__ == "__main__":
m = Solution()
print "s:[aa],p[a] ret:"+str(m.isMatch("aa","a"))
print "s:[aa],p[*] ret:"+str(m.isMatch("aa","*"))
print "s:[cb],p[?a] ret:"+str(m.isMatch("cb","?a"))
print "s:[],p[] ret:"+str(m.isMatch("",""))
print "s:[acdcb],p[a*c?b] ret:"+str(m.isMatch("acdcb","a*c?b"))
print "s:[adceb],p[*a*b] ret:"+str(m.isMatch("adceb","*a*b"))
3. [97. Interleaving String]
3.1 题目
Given s1, s2, s3, find whether s3 is formed by the interleaving of s1 and s2.
Example 1:
Input: s1 = "aabcc", s2 = "dbbca", s3 = "aadbbcbcac"
Output: true
Example 2:
Input: s1 = "aabcc", s2 = "dbbca", s3 = "aadbbbaccc"
Output: false
3.2 思路 && 解题方法
本题的思路还是一样的，维护一个动态数组dp 。dp [ i ] [ j ] 表示 S1 [ i ] 和S2 [ j ] 能匹配 S3 [ i + j - 1] 。假设S1="aa" S2="ab" S3="aaba" , 则从dp [ 0 ] [ 0 ]开始，往右一步代表使用S2 [ j ] 表示S3 [ 0 + j ] ( 0 是 i 因为此处是第一行，所以i是0)。同样往下一步 代表使用S1[ i ] 表示 S3 [ i + 0 ] 。因此如果可以到达dp [ i ] [ j ]则 需要 dp [ i -1 ] [ j ]或者 dp [ i ] [ j -1 ]是1 。
3.3 实现
#!/bin/bash
class Solution(object):
def isInterleave(self, s1, s2, s3):
"""
:type s1: str
:type s2: str
:type s3: str
:rtype: bool
"""
row = len(s1) + 1
col = len(s2) + 1
t = len(s3)
if row + col -2 !=t :
return False
dp = [ [False for j in range(col)] for j in range(row) ]
dp[0][0] = True
for j in range(1,col):
dp[0][j] = dp[0][j-1] and s2[j-1] == s3[j-1]
for i in range(1,row):
dp[i][0] = dp[i-1][0] and s1[i-1] == s3[i-1]
for i in range(1,row):
for j in range(1,col):
dp[i][j] = ( dp[i-1][j] and s1[i-1] == s3[i+j-1]) or (dp[i][j-1] and s2[j-1] == s3[i+j-1])
return dp[row-1][col-1]
if __name__=='__main__':
m = Solution()
print m.isInterleave("a","b","a")
