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https://pintia.cn/problem-sets/1112639514090356736/problems/1112639623511359511

这道题貌似是用递归，大致思路：

double getOp(){
    string str;
    if(str == "+")
        return getOp() + getOp();
    else if(str == "-")
        return getOp() - getOp();
    else if(str == "*")
        return getOp() * getOp();
    else if(str == "/"){
        double a = getOp(), b = getOp();
        if(b == 0) ERROR;//分母不为零 
        else return a / b;
    }
    else return atof(str);
    //字符串转浮点数 
    //如果手写字符串转浮点数，注意 + - 和 . 
}

而只懂后缀表达式的我对Stack有执念：

#include<stdio.h>
#include<string.h>
#include<stdlib.h>
struct node{
    double key;
    node *next = NULL;
}; 
typedef struct node * PtrToNode;
typedef PtrToNode Stack;
Stack creatStack();
void push(Stack s, double t);
bool IsEmpty(Stack s);
void pop(Stack s);
double top(Stack s);    
double ToNumber(char s[]);
int check(Stack s); 
int main()
{
    char s[50][50];
    int i = 0;
    while(scanf("%s", s[i]) != EOF) i++;
    Stack S = creatStack();
    while(i--){
        if(s[i][0] == '+' && strlen(s[i]) == 1){
            if(!check(S)) return 0;
            double a = top(S); pop(S);
            if(!check(S)) return 0;
            double b = top(S); pop(S);
            push(S, a + b);
        }
        else if(s[i][0] == '-' && strlen(s[i]) == 1){
            if(!check(S)) return 0;
            double a = top(S); pop(S);
            if(!check(S)) return 0;
            double b = top(S); pop(S);
            push(S, a - b);
        }
        else if(s[i][0] == '*'){
            if(!check(S)) return 0;
            double a = top(S); pop(S);
            if(!check(S)) return 0;
            double b = top(S); pop(S);
            push(S, a * b);
        }
        else if(s[i][0] == '/'){
            if(!check(S)) return 0;
            double a = top(S); pop(S);
            if(!check(S)) return 0;
            double b = top(S); pop(S);
            if(b == 0){//分母不为零 
                printf("ERROR");
                return 0;
            }
            push(S, a / b);
        }
        else push(S, ToNumber(s[i]));
    }
    if(!check(S)) return 0;
    if(S -> next -> next != NULL) printf("ERROR");
    else printf("%.1lf", top(S));
    return 0;
} 

Stack creatStack(){
    Stack s = (Stack)malloc(sizeof(Stack));
    //while(!IsEmpty(s)) pop(s);
    s -> next = NULL;
    return s;
}
void push(Stack s, double t){
    PtrToNode temp = (Stack)malloc(sizeof(Stack));
    temp -> key = t;
    temp -> next = s -> next;
    s -> next = temp;
}
bool IsEmpty(Stack s){
    return s -> next == NULL;
}
void pop(Stack s){
    PtrToNode first = s -> next;
    s -> next = s -> next -> next;
    free(first);
}
double top(Stack s){
    return s -> next -> key; 
}
double ToNumber(char s[]){
    double sum = 0;
    int len = strlen(s), i = 0, mark = 1;
    if(s[0] == '-'){
        mark = -1; i = 1;
    }
    if(s[0] == '+'){
        mark = 1; i = 1;
    }//zz正号 
    while(i < len && s[i] != '.'){
        sum = sum * 10 +  s[i] - '0';
        i++;
    }
    if(i == len) return sum * mark;
    else{//小数点真有趣 
        i++;
        double p = 10.0;
        while(i < len){
            sum = sum + (s[i] - '0') / p;
            p = p * 10.0;
            i++;
        }
    }
    return sum * mark;
}
int check(Stack s){
    if(IsEmpty(s)){
        printf("ERROR");
        return 0;
    }
    return 1;
}

正负号，小数点，分母不为零。。。真TM坑（手动黑脸

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但AC也是贼jb爽啊hhhhhhh