public int minDistance(String word1, String word2) {
            assert word1 != null && word2 != null;
            int[][] dp = new int[word1.length() + 1][word2.length() + 1];
            for (int i = 0; i <= word2.length(); i++) {
                dp[0][i] = i;
            }
            for (int i = 1; i <= word1.length(); i++) {
                dp[i][0] = i;
                for (int j = 1; j <= word2.length(); j++) {
                    dp[i][j] = Math.min(dp[i - 1][j], dp[i][j - 1]) + 1;
                    if (word1.charAt(i - 1) == word2.charAt(j - 1)) {
                        dp[i][j] = Math.min(dp[i][j], dp[i - 1][j - 1]);
                    } else {
                        dp[i][j] = Math.min(dp[i][j], dp[i - 1][j - 1] + 1);
                    }
                }
            }
            return dp[word1.length()][word2.length()];
        }