test1.c

typedef struct tnode{
    int flag;   //indicate whether the node is a leaf(0: number; 1: variable) or internal node(2: operator)
    char *varname;      //name of the variable
    int val;    //value of the expression tree
    char *op;   //operator branch
    struct tnode *left, *right;     //left and right node
}tnode;