直接枚举

#include<iostream>
#include<cstring>
#include<cstring>

using namespace std;

int get(int x)
{
    if(x<=3500) return x;
    int a[] = {0,1500,4500,9000,35000,55000,80000,1000000};
    int b[] = {0,3,10,20,25,30,35,45};

    x-=3500;
    int tax = 0;
    for(int i = 0; i<8; i++)
        if(x>=a[i-1])
            tax += (min(x,a[i])-a[i-1])/100*b[i];
    return x+=3500-tax;
}

int main()
{
    int t;
    cin>>t;
    //已知小明这个月税后所得为 TT 元，请问他的税前工资 SS 是多少元。
    for(int i = 0; ; i+=100)
        if(get(i) == t)
        {
            cout<<i<<endl;
            break;
        }
    return 0;
}

二分

#include<iostream>
#include<cstring>
#include<cstring>

using namespace std;

int get(int x)
{
    if(x<=3500) return x;
    int a[] = {0,1500,4500,9000,35000,55000,80000,1000000};
    double b[] = {0,0.03,0.10,0.20,0.25,0.30,0.35,0.45};

    if(x<=3500) return x;
    int tax = x-3500;
    double sum = 0;
    for(int i = 0; i<8; i++)
    {
        if(tax>=a[i]) sum += (a[i]-a[i-1])*b[i];
        else 
        {
            sum += (tax - a[i-1])*b[i];
            break;
        }
    }
    return x-sum;
}

int main()
{
    int t;
    cin>>t;
    //已知小明这个月税后所得为 TT 元，请问他的税前工资 SS 是多少元。
    int l = t,r = t*2;
    while(l<r)
    {
        int mid = (l+r)>>1;
        if(get(mid) >= t) r = mid;
        else l = mid+1;
    }
    cout<<l;
    return 0;
}

二分（+0.5）四舍五入

#include<iostream>
#include<cstring>
#include<cstring>

using namespace std;

int get(int x)
{
    if(x<=3500) return x;
    int a[] = {0,1500,4500,9000,35000,55000,80000,1000000};
    double b[] = {0,0.03,0.10,0.20,0.25,0.30,0.35,0.45};

    x-=3500;
    int tax = 0;
    for(int i = 0; i<8; i++)
        if(x>=a[i-1])
            tax += (min(x,a[i])-a[i-1])*b[i]+0.5;
    return x+=3500-tax;
}

int main()
{
    int t;
    cin>>t;
    //已知小明这个月税后所得为 TT 元，请问他的税前工资 SS 是多少元。
    int l = t,r = t*2;
    while(l<r)
    {
        int mid = (l+r)>>1;
        if(get(mid) >= t) r = mid;
        else l = mid+1;
    }
    cout<<l;
    return 0;
}