LeetJava/isMatchRegular.java at master · rwei5/LeetJava · GitHub

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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","aa")  true
isMatch("aaa","aa") false
isMatch("aa", "a*")  true
isMatch("aa", ".*")  true
isMatch("ab", ".*")  true
isMatch("aab", "c*a*b")  true
*/

import java.util.*;
public class isMatchRegular {
	public static void main(String[] args) {
		System.out.println(isMatch("aaa","a*"));
	}
	public static boolean isMatch(String s, String p){
		assert(p!=null && (p.length()==0 || p.charAt(0)!='*'));
        if(p.length() == 0)
           return s.length() == 0;
        if(p.length() == 1 || p.charAt(1) != '*'){
        	if(s.length() < 1 || (p.charAt(0) != '.' && p.charAt(0)!=s.charAt(0)))
        		return false;

        	return isMatch(s.substring(1),p.substring(1));
        }
        else{
        	int i = -1;
            while(i<s.length() && (i<0 || p.charAt(0)=='.' || p.charAt(0)==s.charAt(i))){
                if(isMatch(s.substring(i+1),p.substring(2)))
                    return true;
                i++;
            }
            return false;
        }
   	}
}