表达式直接求值——分治做法

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

#define MAXN 101

char s[MAXN];

int calac(int L, int R){
    int i, p = -1, lv = 0;
    int m = 0;
    
    for (i = L; i <= R; i++){
        switch (s[i]){
            case '(': lv++; break;
            case ')': lv--; break;
            case '+':
            case '-':
                if(!lv) p = i; break;
            case '*':
            case '/':
                if(!lv && p == -1) p = i;
                break;
            default:
                m = m * 10 + s[i] - '0';
        }
    }
    
    if (p == -1)
       return s[L] == '(' ? calac(L + 1, R - 1) : m;
    
    int a1 = calac(L, p - 1), a2 = calac(p + 1, R);
   
    switch (s[p]){
        case '+': return a1 + a2;
        case '-': return a1 - a2;
        case '*': return a1 * a2;
        case '/': return a1 / a2;
    }
}

int main(){
    gets(s);
    
    printf("%d
", calac(0, strlen(s) - 1));
    
    system("pause");
    
    return 0;
}