infixe en postfixe

Bonsoir,

Je tente de cree une fonction qui prend en parametre une chaine de caractere et qui retourne sa notation postfixe
Je suis confronte a un probleme de signe quand j envoie a la fonction "7/8-6/9*1"
Voici la fonction
Code:
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t_postfixe      *infixe_to_postfixe(char *str)
{
  t_postfixe    *postfixe = NULL;
  t_stack       *stack = NULL;
 
  int   i;
 
  i = 0;
  while (str[i])
    {
      if (str[i] >= '0' && str[i] <= '9')
          postfixe = push(postfixe, str[i]);
 
      if (str[i] == '+' || str[i] == '-' || str[i] == '*' || str[i] == '/')
        {
          if (stack == NULL)
            stack = push_stack(stack, str[i]);
 
          else
            {
              if (stack->valeur == '+' && str[i] == '*')
                {
                  stack = push_stack(stack, str[i]);
                }
              else if (stack->valeur == '*' && str[i] == '-' || str[i] == '+')
                {
                  while (stack != NULL)
                    {
                      postfixe = push(postfixe, stack->valeur);
                      stack = pop_stack(stack);
                    }
                  stack = push_stack(stack, str[i]);
                }
 
                if (str[i] == '+' || str[i] == '-' && stack->valeur == '*' || stack->valeur == '/')
                {
                  while (stack != NULL)
                    {
                      postfixe = push(postfixe, stack->valeur);
                      stack = pop_stack(stack);
                    }
                  stack = push_stack(stack, str[i]);
                }
 
else if (str[i] == '*' && stack->valeur == '*')
                {
                  postfixe = push(postfixe, stack->valeur);
                  stack = pop_stack(stack);
                  stack = push_stack(stack, str[i]);
                }
 
              else if (str[i] == '/' && stack->valeur != '*')
                {
                  stack = push_stack(stack, str[i]);
                }
 
              else if (str[i] == '*' && stack->valeur == '/')
                {
                  stack = pop_stack(stack);
                  postfixe = push(postfixe, stack->valeur);
                  stack = push_stack(stack, str[i]);
                }
              else if (str[i] == '/' && stack->valeur == '*')
                {
                  postfixe = push(postfixe, stack->valeur);
                  stack = pop_stack(stack);
                  stack = push_stack(stack, str[i]);
                }
              else if (stack->valeur == '-' && str[i] == '/')
	        {
	          stack = push_stack(stack, str[i]);
                }
 
              else if (str[i] == '-' && stack->valeur == '/')
                {
 
                  while (stack != NULL)
                    {
                      postfixe = push(postfixe, stack->valeur);
                      stack = pop_stack(stack);
                    }
                  stack = push_stack(stack, str[i]);
 
                }
 
 
            }
        }
      i++;
    }
En vous remerciant par avance,