Regular Expression Matching

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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).The function prototype should be:bool isMatch(const char *s, const char *p)Some examples:isMatch("aa","a") → falseisMatch("aa","aa") → trueisMatch("aaa","aa") → falseisMatch("aa", "a*") → trueisMatch("aa", ".*") → trueisMatch("ab", ".*") → trueisMatch("aab", "c*a*b") → true

判断正则表达式是否合法正则表达式会用 . 来表示任意一个字符用 *来表示一个或多个上一字符根据这两点在进行判断时最主要是分清 * 存在与否的情况

代码如下：

public class Solution {    public boolean isMatch(String s, String p) {        if(p.length()==0) return s.length()==0;if(p.length()==1){if(s.length()<1||s.charAt(0)!=p.charAt(0)&&p.charAt(0)!='.')return false;else{return isMatch(s.substring(1), p.substring(1));}}if(p.charAt(1)=='*'){if(isMatch(s,p.substring(2))) return true;for(int i=0;i<s.length();i++){    if(p.charAt(0) != s.charAt(i)&&p.charAt(0)!='.')return false;if(isMatch(s.substring(i+1), p.substring(2))) return true;}}else{if(s.length()<1||(p.charAt(0)!='.'&&s.charAt(0)!=p.charAt(0)))return false;else{return isMatch(s.substring(1), p.substring(1));}}return false;    }}

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