Given two words word1 and word2, find the minimum number of steps required to convert word1 to word2. (each operation is counted as 1 step.)

You have the following 3 operations permitted on a word:

a) Insert a character
b) Delete a character
c) Replace a character

O(mn) time O(mn) space DP solution

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public class Solution {
    public int minDistance(String word1, String word2) {
        if (word1 == null || word2 == null) {
            return -1;
        }
        int m = word1.length();
        int n = word2.length();
        int[][] distance = new int[m + 1][n + 1];
        for (int i = 0; i <= m; i++) {
            distance[i][0] = i;
        }
        for (int i = 0; i <= n; i++) {
            distance[0][i] = i;
        }
        
        for (int i = 1; i <= m; i++) {
            for (int j = 1; j <= n; j++) {
                if (word1.charAt(i - 1) == word2.charAt(j - 1)) {
                    distance[i][j] = distance[i - 1][j - 1];
                } else {
                    distance[i][j] = Math.min(Math.min(distance[i - 1][j] + 1, distance[i][j - 1] + 1), distance[i - 1][j - 1] + 1);
                }
            }
        }
        return distance[m][n];
    }
}