Leetcode:reverse Words in a String problem solving report

Reverse Words in a String

Given an input string, reverse the string word by word.

For example,
Given s = "The Sky is Blue",
Return "Blue is Sky the".

Click to show Clarification.

Clarification:
What constitutes a word?
A sequence of non-space characters constitutes a word.
Could the input string contain leading or trailing spaces?
Yes. However, your reversed string should not contain leading or trailing spaces.
How about multiple spaces between and words?
Reduce them to a single space in the reversed string.

Solution 1:

1. First process the original string:

Use Strim to remove the front and back spaces.

2. Use the regular expression "\\s+" to match multiple spaces. So that any number of spaces will match, using split to successfully divide the string into separate words.

Note that \ \ is escaping \\s+ and will get \s+

"+" means 1 or more spaces.

Example:

"I love CMU" such will be directly divided into: I, love, CMU.

Let's take a look at the regular expression:

Http://zh.wikipedia.org/wiki/%E6%AD%A3%E5%88%99%E8%A1%A8%E8%BE%BE%E5%BC%8F

The complete collection of expressions [edit ]

There are many different styles of regular expressions. The following table is a complete list of metacharacters in Pcre and their behavior in the context of regular expressions, for Perl or Python programming languages (the regular expression grammars of grep or egrep are subsets of Pcre):

character	Description
\	Marks the next character as a special character, or a literal character, or a backward reference, or an octal escape. For example, " n " matches the character " n ". " \n " matches a line break. The sequence " \\ matches" \ and "matches" \( ( .
^	Matches the starting position of the input string. If the multiline property of the RegExp object is set, ^ also matches the \n position after "or" \r .
$	Matches the end position of the input string. If the multiline property of the RegExp object is set, $ also matches the \n position before "" or "" \r .
*	Matches the preceding subexpression 0 or more times. For example, zo* can match " z ," zo "and" zoo ". * Equivalent to {0,}.
+	Matches the preceding subexpression one or more times. For example, " zo+ can match" zo and " zoo , but cannot match" " z . + equivalent to {1,}.
?	Matches the preceding subexpression 0 or one time. For example, "" can Match "" or "" do(es)? do in "." does is do equivalent to {0,1}.
N	N is a non-negative integer. Matches the determined n times. For example, "" o{2} cannot Match "" Bob in " o , but can match food two o in" ".
{N,}	N is a non-negative integer. Match at least n times. For example, "" o{2,} cannot Match " Bob in" o , but can match foooood all o in "". "" is o{1,} equivalent to " o+ ". "" o{0,} is equivalent to " o* ".
{N,m}	Both M and n are non-negative integers, where n<=m. Matches at least n times and matches up to M times. For example, "" o{1,3} will match the fooooood top three o in "". "" is o{0,1} equivalent to " o? ". Note that there can be no spaces between a comma and two numbers.
?	When the character immediately follows any other restriction (*,+,?,{n},{n,},{n,m}), the matching pattern is non-greedy. The non-greedy pattern matches the searched string as little as possible, while the default greedy pattern matches as many of the searched strings as possible. For example, for the string " oooo ", "" o+? will match a single "" and "" o o+ will match all " o ".
.	Matches \ n any single character except "". To match \ n any character including "", use (.|\n) a pattern like "".
(pattern)	Matches the pattern and obtains this matching substring. This substring is used for backward referencing. The obtained matches can be obtained from the resulting matches collection, the Submatches collection is used in VBScript, and the $0...$9 property is used in JScript. To match the parentheses character, use " \( or" \) .
(?:p Attern)	Matches the pattern but does not get a matching substring, which means that this is a non-fetch match and does not store matching substrings for backward reference. This is useful in using the or character " (|) " to combine the various parts of a pattern. For example, " industr(?:y|ies) " is a industry|industries more abbreviated expression than "".
(? =pattern)	Positive pre-check to match the find string at the beginning of any string matching pattern. This is a non-fetch match, which means that the match does not need to be acquired for later use. For example, " Windows(?=95|98|NT|2000) can match" Windows2000 in Windows , but not " Windows3.1 in" Windows . Pre-checking does not consume characters, that is, after a match occurs, the next matching search starts immediately after the last match, rather than starting with the character that contains the pre-check.
(?! Pattern	Forward negation, matching the lookup string at the beginning of any mismatched pattern string. This is a non-fetch match, which means that the match does not need to be acquired for later use. For example Windows(?!95|98|NT|2000) , "can match" Windows3.1 in Windows , but cannot match "" Windows2000 in Windows "". Pre-check does not consume characters, that is, after a match occurs, the next matching search starts immediately after the last match, rather than starting with the character that contains the pre-check
(? <=pattern)	Reverse positive pre-check, similar to positive pre-check, just the opposite direction. For example, " (?<=95|98|NT|2000)Windows can match" 2000Windows in Windows , but not " 3.1Windows in" Windows .
(? <!pattern)	Reverse negation is similar to positive negative pre-checking, except in the opposite direction. For example (?<!95|98|NT|2000)Windows , "can match" 3.1Windows in Windows , but cannot match "" 2000Windows in Windows "".
X|y	Match x or Y. For example, " z|food can match" z or " food ." "" matches "" or "" (z|f)ood zood food .
[XYZ]	Character set (character Class). Matches any one of the characters contained. For example, "" can Match "" in "" [abc] plain a . Special characters have only backslashes \ Hold special meanings and are used to escape characters. Other special characters Furus, plus, all kinds of parentheses are used as ordinary characters. Caret ^ If it appears in the first place, it represents a negative character set, and if it appears in the middle of a string, it is only ordinary characters. Hyphen-Represents a character range description if it appears in the middle of a string, or only as a normal character if it appears in the first place.
[^XYZ]	Exclusion type (negate) character set. Matches any character that is not listed. For example, "" can Match "" in "" [^abc] plain plin .
[A-z]	The character range. Matches any character within the specified range. For example, " [a-z] " can match a z any lowercase alphabetic character in the "to" range.
[^a-z]	The character range of the exclusion type. Matches any character that is not in the specified range. For example, " [^a-z] " can match any character that is not in the " a to" z range.
\b	Matches a word boundary, which is the position between a word and a space. For example, "" can Match "in" er\b never er , but not " verb in" er .
\b	Matches a non-word boundary. "" Can Match "in" er\B verb er , but not " never in" er .
\cx	Matches the control character indicated by X. For example, \cm matches a control-m or carriage return. The value of x must be one of a-Z or a-Z. Otherwise, c is considered to be a literal " c " character.
\d	Matches a numeric character. equivalent to [0-9].
\d	Matches a non-numeric character. equivalent to [^0-9].
\f	Matches a page break. Equivalent to \x0c and \CL.
\ n	Matches a line break. Equivalent to \x0a and \CJ.
\ r	Matches a carriage return character. Equivalent to \x0d and \cm.
\s	Matches any whitespace character, including spaces, tabs, page breaks, and so on. equivalent to [\f\n\r\t\v].
\s	Matches any non-whitespace character. equivalent to [^ \f\n\r\t\v].
\ t	Matches a tab character. Equivalent to \x09 and \ci.
\v	Matches a vertical tab. Equivalent to \x0b and \ck.
\w	Matches any word character that includes an underscore. Equivalent to " [A-Za-z0-9_] ".
\w	Matches any non-word character. Equivalent to " [^A-Za-z0-9_] ".
\xn	Match N, where n is the hexadecimal escape value. The hexadecimal escape value must be two digits long for a determination. For example, " \x41 match" A . "" \x041 is equivalent to " \x04&1 ". ASCII encoding can be used in regular expressions:
\num	Backward References (back-reference) a substring (substring) that matches the first num of the regular expression with parentheses around the subexpression (subexpression). Where num is a positive integer starting at 1, the upper limit may be 99. For example: " (.)\1 " matches two consecutive identical characters.
\ n	Identifies an octal escape value or a backward reference. n is a backward reference if \ n is preceded by at least one of the sub-expressions obtained. Otherwise, if n is the octal number (0-7), N is an octal escape value.
\nm	Identifies an octal escape value or a backward reference. If at least NM has obtained a subexpression before \nm, then NM is a backward reference. If there are at least N fetches before \nm, then n is a backward reference followed by the literal m. If none of the preceding conditions are met, if both N and M are octal digits (0-7), then \nm will match the octal escape value nm.
\nml	If n is an octal number (0-3) and both M and L are octal digits (0-7), the octal escape value NML is matched.
\un	Match N, where N is a Unicode character represented by four hexadecimal digits. For example, \u00a9 matches the copyright symbol (©).

priority [edit ]

Priority	symbols
Highest	\
High	(), (?:), (? =), []
In	*, + 、?、 {n}, {n,}, {m,n}
Low	^, $, intermediary characters
Minimum	L

3. Scan backward, add a word, plus a space, the last time not added.

1  Public classSolution {2      Publicstring Reversewords (string s) {3         if(s = =NULL) {4             return NULL;5         }6         7StringBuilder SB =NewStringBuilder ();8         9         //Remove the leading and the tailing string.TenString snew =S.trim (); Onestring[] STRs = Snew.split ("\\s+"); A          -         //Construct the new string. -          for(inti = strs.length-1; I >= 0; i--) { the sb.append (Strs[i]); -             if(I! = 0) { -Sb.append (""); -             } +         } -          +         returnsb.tostring (); A     } at}

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Leetcode:reverse Words in a String problem solving report