longestCommonSubsequence

public static List<Integer> longestCommonSubsequence(List<Integer> a, List<Integer> b) {
List<Integer> ans = new ArrayList<>();
int m = a.size() + 1;
int n = b.size() + 1;
int[][] arr = new int[m][n];
if (a.size() < 0 || b.size() < 0) {
return ans;
}
for (int i = 1; i <= m - 1; i++) {
for (int j = 1; j <= n - 1; j++) {
if (a.get(i - 1) != null && b.get(j - 1) != null && a.get(i - 1) == b.get(j - 1)) {
arr[i][j] = arr[i - 1][j - 1] + 1;
} else {
arr[i][j] = Math.max(arr[i - 1][j], arr[i][j - 1]);
}
}
}
int i = a.size();
int j = b.size();
while (i > 0 && j > 0) {
if (a.get(i - 1) != null && b.get(j - 1) != null && a.get(i - 1) == b.get(j - 1)) {
ans.add(a.get(i - 1));
i--;
j--;
} else if (arr[i][j] == arr[i][j - 1]) {
j--;
} else {
i--;
}
}
Collections.reverse(ans);
return ans;
}