1092. Shortest Common Supersequence

class Solution {
    public void helper(String s1, String s2, int[][] dp) {
        int n = s1.length();
        int m = s2.length();
        for(int i = 1; i <= n; i++) {
            for(int j = 1; j <= m; j++) {
                if(s1.charAt(i - 1) == s2.charAt(j - 1)) {
                    dp[i][j] = 1 + dp[i - 1][j - 1];
                }
                else {
                    dp[i][j] = Math.max(dp[i - 1][j], dp[i][j - 1]);
                }
            }
        }
    }
    public String shortestCommonSupersequence(String str1, String str2) {
        int[][] dp = new int[str1.length() + 1][str2.length() + 1];
        helper(str1, str2, dp);
        int i = str1.length();
        int j = str2.length();
        String ans = "";
        while (i > 0 && j > 0) {
            if(str1.charAt(i - 1) == str2.charAt(j - 1)) {
                ans += str1.charAt(i - 1);
                i--;
                j--;
            }
            else if(dp[i - 1][j] > dp[i][j - 1]) {
                ans += str1.charAt(i - 1);
                i--;
            }
            else {
                ans += str2.charAt(j - 1);
                j--;
            }
        }
        while (i > 0) {
            ans += str1.charAt(i - 1);
            i--;
        }
        while (j > 0) {
            ans += str2.charAt(j - 1);
            j--;
        }
        String ans1 = "";
        for(int k = ans.length() - 1; k >= 0; k--) {
            ans1 += ans.charAt(k);
        }
        return ans1;
    }
}