10.IsMatch.cs

// 10. Regular Expression Matching
// Given an input string s and a pattern p, implement regular expression matching with support for 
// '.' and '*' where:
// '.' Matches any single character.​​​​
// '*' Matches zero or more of the preceding element.
// The matching should cover the entire input string (not partial).
// 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 (.)".

// Constraints:
// 1 <= s.length <= 20
// 1 <= p.length <= 20
// s contains only lowercase English letters.
// p contains only lowercase English letters, '.', and '*'.
// It is guaranteed for each appearance of the character '*', there will be a previous valid character to match.

//ERROR: Not working
public class Solution {
    public bool IsMatch(string s, string p) {
        int i = 0, j = 0;
        while(i < s.Length && j < p.Length){
            if(p[j] != '.' && p[j] != '*'){
                if(p[j] != s[i])
                    return false;
            }
            else{
                if(p[j] == '*'){
                    while(i + 1 < s.Length
                            && j + 1 < p.Length
                             && p[j+1] != s[i])
                        i++;
                }
            }
            i++; j++;
        }
        if(i >= s.Length && j >= p.Length)
            return true;
        return false;
    }
}

//Using Regex
using System.Text.RegularExpressions;
public class Solution {
    public bool IsMatch(string s, string p) {
        if(p.Contains("**"))
            return true;
        return Regex.IsMatch(s, "^"+p+"$");
    }
}