test.c

#include "stdio.h"

typedef struct tree tree;
struct tree{
   int value;
   tree *left ;
   tree *right;
};

tree *tree_first_bigger(tree *t) {
    if (t->left != NULL)
        return tree_first_bigger(t->left);
    return t;
}

tree *find_first_bigger(tree *t, int x){
   if(t == NULL)
      return NULL;

   if(t->value > x) {
      tree *answer = find_first_bigger(t->left, x);
      if (answer != NULL) return answer;
      return t;
   } else if(t->value < x) {
      return find_first_bigger(t->right, x);
   } else if(t->value == x) {
      if (t->right != NULL)
          return tree_first_bigger(t->right);
      return NULL;
   }
}  

void setT(tree *t, int value, tree* left, tree* right) {
    t->value = value;
    t->left = left;
    t->right = right;
}

int main() {
    tree T, J, U, O, P, M, N;
    setT(&T, 'T', &J, &U);
    setT(&J, 'J', NULL, &O);
    setT(&U, 'U', NULL, NULL);
    setT(&O, 'O', &M, &P);
    setT(&P, 'P', NULL, NULL);
    setT(&M, 'M', NULL, &N);
    setT(&N, 'N', NULL, NULL);

    printf("%c\n", (char)find_first_bigger(&T, 'T')->value);
    printf("%c\n", (char)find_first_bigger(&T, 'N')->value);
    printf("%c\n", (char)find_first_bigger(&T, 'P')->value);

    return 0;
}