I came across this problem that asks you to implement a regular expression matcher with support for '.' and '*', where
'.' Matches any single character.
'*' Matches zero or more of the preceding element.
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
While I was able to solve this in a linear fashion, I came across lots of solutions that used DP, like the one below,
class Solution {
public boolean isMatch(String text, String pattern) {
boolean[][] dp = new boolean[text.length() + 1][pattern.length() + 1];
dp[text.length()][pattern.length()] = true;
for (int i = text.length(); i >= 0; i--){
for (int j = pattern.length() - 1; j >= 0; j--){
boolean first_match = (i < text.length() &&
(pattern.charAt(j) == text.charAt(i) ||
pattern.charAt(j) == '.'));
if (j + 1 < pattern.length() && pattern.charAt(j+1) == '*'){
dp[i][j] = dp[i][j+2] || first_match && dp[i+1][j];
} else {
dp[i][j] = first_match && dp[i+1][j+1];
}
}
}
return dp[0][0];
}
}
I'm having a hard time understanding this. I've solved a few DP problems that involved grids (shortest path in 2d grid, largest square in binary 2d grid), using a DP table there made perfect sense to me. However here I'm completely lost, I'm unable to understand how traversing a 2d table helps in solving this problem. Further more it appears we know when the characters don't match in the loop, so I don't understand why we don't terminate the search there (this is also probably due to my lack of understanding of how a table traversal leads to a solution). Is there a clear intuitive explanation for problems like these?