Language: C

char s[3333],t[3333];
int dp[3333][3333];
i,j;
ans(i,j){
	i&&j?s[i-1]==t[j-1]?ans(i-1,j-1),putchar(s[i-1]):dp[i][j]==dp[i-1][j]?ans(i-1,j):ans(i,j-1):0;
}
main(l)
{
	gets(s);l=strlen(gets(t));
	for(i=0;i<strlen(s);i++)
	{
		for(j=0;j<l;j++)
		{
			dp[i+1][j+1]=s[i]-t[j]?fmax(dp[i+1][j],dp[i][j+1]):dp[i][j]+1;
		}
	}
	ans(i,j);puchar(0);
}

Compiler output:
main.c:3:1: warning: data definition has no type or storage class
    3 | i,j;
      | ^
main.c:3:1: error: type defaults to 'int' in declaration of 'i' [-Wimplicit-int]
main.c:3:3: error: type defaults to 'int' in declaration of 'j' [-Wimplicit-int]
    3 | i,j;
      |   ^
main.c:4:1: error: return type defaults to 'int' [-Wimplicit-int]
    4 | ans(i,j){
      | ^~~
main.c: In function 'ans':
main.c:4:1: error: type of 'i' defaults to 'int' [-Wimplicit-int]
main.c:4:1: error: type of 'j' defaults to 'int' [-Wimplicit-int]
main.c:5:42: error: implicit declaration of function 'putchar' [-Wimplicit-function-declaration]
    5 |         i&&j?s[i-1]==t[j-1]?ans(i-1,j-1),putchar(s[i-1]):dp[i][j]==dp[i-1][j]?ans(i-1,j):ans(i,j-1):0;
      |                                          ^~~~~~~
main.c:1:1: note: include '<stdio.h>' or provide a declaration of 'putchar'
  +++ |+#include <stdio.h>
    1 | char s[3333],t[3333];
main.c: At top level:
main.c:7:1: error: return type defaults to 'int' [-Wimplicit-int]
    7 | main(l)
      | ^~~~
main.c: In function 'main':
main.c:7:1: error: type of 'l' defaults to 'int' [-Wimplicit-int]
main.c:9:9: error: implicit declaration of function 'gets' [-Wimplicit-function-declaration]
    9 |         gets(s);l=strlen(gets(t));
      |         ^~~~
main.c:9:19: error: implicit declaration of function 'strlen' [-Wimplicit-function-declaration]
    9 |         gets(s);l=strlen(gets(t));
      |                   ^~~~~~
main.c:1:1: note: include '<string.h>' or provide a declaration of 'strlen'
  +++ |+#include <string.h>
    1 | char s[3333],t[3333];
main.c:9:19: warning: incompatible implicit declaration of built-in function 'strlen' [-Wbuiltin-declaration-mismatch]
    9 |         gets(s);l=strlen(gets(t));
      |                   ^~~~~~
main.c:9:19: note: include '<string.h>' or provide a declaration of 'strlen'
main.c:9:26: error: passing argument 1 of 'strlen' makes pointer from integer without a cast [-Wint-conversion]
    9 |         gets(s);l=strlen(gets(t));
      |                          ^~~~~~~
      |                          |
      |                          int
main.c:9:26: note: expected 'const char *' but argument is of type 'int'
main.c:14:48: error: implicit declaration of function 'fmax' [-Wimplicit-function-declaration]
   14 |                         dp[i+1][j+1]=s[i]-t[j]?fmax(dp[i+1][j],dp[i][j+1]):dp[i][j]+1;
      |                                                ^~~~
main.c:1:1: note: include '<math.h>' or provide a declaration of 'fmax'
  +++ |+#include <math.h>
    1 | char s[3333],t[3333];
main.c:14:48: warning: incompatible implicit declaration of built-in function 'fmax' [-Wbuiltin-declaration-mismatch]
   14 |                         dp[i+1][j+1]=s[i]-t[j]?fmax(dp[i+1][j],dp[i][j+1]):dp[i][j]+1;
      |                                                ^~~~
main.c:14:48: note: include '<math.h>' or provide a declaration of 'fmax'
main.c:17:18: error: implicit declaration of function 'puchar' [-Wimplicit-function-declaration]
   17 |         ans(i,j);puchar(0);
      |                  ^~~~~~