Language: C

#include<stdio.h>
#define N 26

void pre(int);
void ino(int);
void pos(int);
??
struct Node{
 int v, a, b; 
};

struct Node G[N];

int n, i, l, r, p;

char S[4][100]={"Preorder","Inorder","Postorder"};

int main(){
????????void (*funcs[])(int)={pre,ino,pos};
????????scanf("%d",&n);
????????for(i=0;i<n;i++)
????????{
????????????????scanf("%d%d%d",&p,&l,&r);
????????????????G[l].v=G[r].v=1;
????????????????G[p].a=l;
????????????????G[p].b=r;
????????}
????????for(i=0;i<n;i++)if(!G[i].v)p=i;
????????for(i=0;i<3;i++)
????????{
????????????????puts(S[i]);
????????????????(*(funcs[i]))(p);
????????????????puts("");
????????}
????????return 0;
}

void pre(int n)
{
????????if(n==-1)return;
????????printf(" %d",n);
????????pre(G[n].a);
????????pre(G[n].b);
????????return;
}

void ino(int n)
{
????????if(n==-1)return;
????????ino(G[n].a);
????????printf(" %d",n);
????????ino(G[n].b);
????????return;
}

void pos(int n)
{
????????if(n==-1)return;
????????pos(G[n].a);
????????pos(G[n].b);
????????printf(" %d",n);
????????return;
}

Compiler output:
main.c:7:1: error: expected identifier or '(' before '?' token
    7 | ??
      | ^
main.c:12:13: error: array type has incomplete element type 'struct Node'
   12 | struct Node G[N];
      |             ^
main.c: In function 'main':
main.c:19:1: error: expected expression before '?' token
   19 | ????????void (*funcs[])(int)={pre,ino,pos};
      | ^
main.c:20:1: error: expected expression before '?' token
   20 | ????????scanf("%d",&n);
      | ^
main.c:21:1: error: expected expression before '?' token
   21 | ????????for(i=0;i<n;i++)
      | ^
main.c:29:1: error: expected expression before '?' token
   29 | ????????for(i=0;i<3;i++)
      | ^
main.c:32:15: warning: trigraph ??( ignored, use -trigraphs to enable [-Wtrigraphs]
   32 | ????????????????(*(funcs[i]))(p);
main.c: In function 'pre':
main.c:40:1: error: expected expression before '?' token
   40 | ????????if(n==-1)return;
      | ^
main.c:41:1: error: expected expression before '?' token
   41 | ????????printf(" %d",n);
      | ^
main.c:42:1: error: expected expression before '?' token
   42 | ????????pre(G[n].a);
      | ^
main.c:43:1: error: expected expression before '?' token
   43 | ????????pre(G[n].b);
      | ^
main.c:44:1: error: expected expression before '?' token
   44 | ????????return;
      | ^
main.c: In function 'ino':
main.c:49:1: error: expected expression before '?' token
   49 | ????????if(n==-1)return;
      | ^
main.c:50:1: error: expected expression before '?' token
   50 | ????????ino(G[n].a);
      | ^
main.c:51:1: error: expected expression before '?' token
   51 | ????????printf(" %d",n);
      | ^
main.c:52:1: error: expected expression before '?' token
   52 | ????????ino(G[n].b);
      | ^
main.c:53:1: error: expected expression before '?' token
   53 | ????????return;
      | ^
main.c: In function 'pos':
main.c:58:1: error: expected expression before '?' token
   58 | ????????if(n==-1)return;
      | ^
main.c:59:1: error: expected expression before '?' token
   59 | ????????pos(G[n].a);
      | ^
main.c:60:1: error: expected expression before '?' token
   60 | ????????pos(G[n].b);
      | ^
main.c:61:1: error: expected expression before '?' token
   61 | ????????printf(" %d",n);
      | ^
main.c:62:1: error: expected expression before '?' token
   62 | ????????return;
      | ^