题目前置知识

c++中的string判空

string Count;
Count.empty(); //正确
Count != null; //错误

c++中最大最小宏

#include <limits.h>

INT_MAX
INT_MIN

字符串使用+发运算将字符加到字符串末尾

string Count;
string str = "liuda";
Count += str[i];

 题目概况

AC代码

#include <iostream>
#include <cstring>
#include <limits.h>
using namespace std;

long long int count = 0;
string Count;
int findCount(string &str){
	int bit = 1;
	int n=str.size();
    for(int i=0;i<n;i++){
        if(str[i]>='0'&&str[i]<='9'){
            Count += str[i];
        }else if(!Count.empty()){
        	break;
		}
    }
    if(!Count.empty()){
    for(int i=Count.size()-1;i>=0;i--){
        count += (Count[i]-'0')*bit;
		bit = bit * 10;	
	} 
//这里如果题目有负数的话，就不能这么做了。
	 if(count<=INT_MAX&&count>=0){
       return count;
    }
    }

    return -1;
}

int main(){
    string str;
    cin>>str;
    int result=findCount(str);
    cout<<result;
    return 0;
}