How to convert from NPN expression to tree?

Question

I have been developing about NPN expressions in C. The problem is that I should create I function which takes a string and creates the tree according to these examples: enter image description here

I have started the function, using strtok in order to take the differents token of the string:

T_Tree readgenerate(char string []) {
    char *token = strtok(string," ");
    double value;
    T_Tree tree;
    while(token != NULL) {
        token = strtok(NULL," ");
        char disc = token[0];
        tree->discriminant = disc;
        if(disc == '*' || disc == '+' || disc == '-' || disc == '/' || disc == 'p') {
            tree = insertBoth(disc, NULL, NULL); 
        }
    }
}

However, I dont know how to complete it, and I would appreciatte u to help me.

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