Question

I'm trying to create a binary tree like this: link

enter image description here

The input will be entered by the user as prefix and read as a string, then put in a binary tree.

This is what I have so far:

struct node{
   char val;
   struct node *left;
   struct node *right;
};
typedef struct node root;
typedef root *tree;

In main:

void main(){
    int i;
    tree tr;
    char* s;
    s=input(); //input function
    tr=create_empty_tree(); 
    for(i=0;s[i]!='\0';i++){
        tr=add_root(s[i],ab);
    }
    convert_infix(tr);
}

and this is the part I've been struggling with for days; I can't seem to build the tree correctly. This is what I have so far:

tree add_root(char val, tree tr){
    if ( tr == NULL ){
        tr= create_root(val);
    }
    else{
        if(val=="*" || val=="+" || val=="-" || val=="/"){
            tr->left= add_root(val, tr->left);
        }
        else{
            if(tr->left == NULL){
                tr->left= add_root(val, tr->left);
            }
            else{
                tr->right= add_root(val, tr->right);
            }
        }
    }
    return tr;
}

I looked this up online and I know that my function is wrong, I tried doing this:

Insert new nodes, each time moving to the left until an operand has been inserted.
Backtrack to the last operator, and put the next node to its right.
Continue in the same pattern.

but I don't know how to backtrack. I've been at this for days and I'm going crazy; I'm just a beginner in programming so please don't judge.

Answer 1

您在此处遇到问题，char不是字符串char*，因此您无法在char和char *之间进行比较。 ..... 当''用于c字符串，""时，char *用于char，请替换为：

    if(val=="*" || val=="+" || val=="-" || val=="/")

通过以下代码

    if(val=='*' || val=='+' || val=='-' || val=='/')

Answer 2

简单样本（简化了错误检查）

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>

struct node{
   char op;
   int val;
   struct node *left;
   struct node *right;
};

typedef struct node Tree;

char *get_token(char **p){
    while(**p && isspace(**p)){//skip spaces
        ++*p;
    }
    char *ret = *p;
    if(!*ret){//empty
        return NULL;
    }
    while(**p && !isspace(**p)){
        ++*p;
    }
    if(!**p){
        **p = 0;
        ++*p;
    }
    return ret;
}

Tree *make_tree(char **p){
    char *token = get_token(p);

    if(token){
        Tree *tree = malloc(sizeof(*tree));

        switch(*token){//token[1] == '\0' ??
        case '*':
        case '/':
        case '+':
        case '-':
            tree->op = *token;
            tree->val = 0;
            tree->left = make_tree(p);
            tree->right = make_tree(p);
            break;
        default:
            tree->op = 'N';
            tree->val = atoi(token);
            tree->left = tree->right = NULL;
        }
        return tree;
    }
    return NULL;
}

void release(Tree *tree){
    if(tree){
        release(tree->left);
        release(tree->right);
        free(tree);
    }
}

void print_infix(Tree *tree){
    if(tree){
        switch(tree->op){
        case '*':
        case '/':
        case '+':
        case '-':
            putchar('(');print_infix(tree->left);printf(" %c ", tree->op);print_infix(tree->right);putchar(')');
            break;
        case 'N':
            printf("%d", tree->val);
            break;
        default:
            fprintf(stderr, "\nerror!\n");//this should never be executed
        }
    }
}

int main(void){
    char line[256];

    while(fgets(line, sizeof(line), stdin)){
        char *s = line;
        Tree *tree = make_tree(&s);
        print_infix(tree);
        putchar('\n');
        release(tree);
    }
    return 0;
}
#if 0
* + 7 3 - 5 2
((7 + 3) * (5 - 2))
* 3 + 1  2
(3 * (1 + 2))
/ + 1 + 2 3 5
((1 + (2 + 3)) / 5)
^Z
#endif

Building parse tree from prefixed expression in c

2 个答案: