10. 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”) → 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

public class Solution {    public boolean isMatch(String s, String p) {        // base case        if (p.length() == 0) {            return s.length() == 0;        }        // special case        if (p.length() == 1) {            // if the length of s is 0, return false            if (s.length() < 1) {                return false;            }            // if the first does not match, return false            else if ((p.charAt(0) != s.charAt(0)) && (p.charAt(0) != '.')) {                return false;            }            // otherwise, compare the rest of the string of s and p.            else {                return isMatch(s.substring(1), p.substring(1));            }        }        // case 1: when the second char of p is not '*'        if (p.charAt(1) != '*') {            if (s.length() < 1) {                return false;            }            if ((p.charAt(0) != s.charAt(0)) && (p.charAt(0) != '.')) {                return false;            } else {                return isMatch(s.substring(1), p.substring(1));            }        }        // case 2: when the second char of p is '*', complex case.        else {            // case 2.1: a char & '*' can stand for 0 element            if (isMatch(s, p.substring(2))) {                return true;            }            // case 2.2: a char & '*' can stand for 1 or more preceding element,            // so try every sub string            int i = 0;            while (i < s.length()                    && (s.charAt(i) == p.charAt(0) || p.charAt(0) == '.')) {                if (isMatch(s.substring(i + 1), p.substring(2))) {                    return true;                }                i++;            }            return false;        }    }}

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