leetcode 10 regular expression matching

leetcode 10 regular expression matching

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:

• s could be empty and contains only lowercase letters a-z.
• p could be empty and contains only lowercase letters a-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 preceding 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

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solution one

recursion

class Solution {
    public boolean isMatch(String s, String p) {
        if(p.isEmpty())
            return s.isEmpty();
        boolean first_match = (!p.isEmpty() && (p.charAt(0) == s.charAt(0) || 
                              p.charAt(0) == '.'));
        
        if(p.length >=2 && p.charAt(1) == '*') {
            return (isMatch(s, p.substring(2)) || 
                    (first_match && isMatch(s.substring(1),p)));
        }
        else {
            return first_match && isMatch(s.substring(1), p.substring(1));
        }
    }
}