[LeetCode] Regular Expression Matching

Problems:

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

Solutions:O(n)

public class Solution {
    public boolean isMatch(String s, String p) {
        if(s == null || p == null)
            return false;
        
        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 len = s.length();
            int i=-1;
            while(i<len && (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;
        }    
    }
}

Key Concept: if * is the second place, we need to consider whole case else easy to move forward.