计算代码

class Solution {
public:
    int getSum(int a, int b) {
        double base = 2200000;//2160269.31;
        double ratio = 0.04165;
        int totalMonths = 360;
        double monthlyPayout = base * (ratio/12) * pow((1 + ratio/12), totalMonths) / (pow((1 + ratio/12), totalMonths) - 1);
        double totalPayout = monthlyPayout * totalMonths;
        //cout << monthlyPayout <<endl;
        
        int monthIndex = 1;
        // 在第indexByAdvance个月决定还款
        int indexByAdvance = 24;
        
        bool flag = false;
        
        // 提前还款的额度
        double x = 200000;
        // 假设当前投资别的东西年收益率
        double ratioByInvest = 0.045;
        
        // 不提前还款，到第monthIndex个月为止的总利息
        double sumInterest = 0;
        
        // 不提前还款，到第monthIndex个月为止的剩余本金
        double leftBase = base;
        
        // 不提前还款剩余的总利息总和
        double leftInterest = monthlyPayout*totalMonths - base;
        
        // 提前还款剩余的总利息总和
        
        // 提前还款（设在第K个月提前还款x元，K<monthIndex），到第monthIndex个月为止的总利息
        double sumInterestByAdvance = 0;
        
        // 提前还款（设在第K个月提前还款x元，K<monthIndex），到第monthIndex个月为止的剩余本金
        double leftBaseByAdvance = base;
        
        // x元做投资的总回报
        double paybackByInvest = 0;
        
        
        
        while(monthIndex <= totalMonths && leftBaseByAdvance >=0) {
            /*
              计算到第monthIndex个月为止的总支出（查看剩余本金和已经支出的利息）:
              1. 假设不提前还款
              （1）到第monthIndex个月为止的总利息 = monthlyPayout * monthIndex;
              2. 在第K个月提前还款X元的总支出 = base - monthlyPayout * (monthIndex - k) - ()
            */
            
            if (monthIndex == indexByAdvance) {
                leftBaseByAdvance = leftBaseByAdvance - x;
            }
            
            if (monthIndex >= indexByAdvance) {
                double cInterest = leftBaseByAdvance * (ratio/12);
                double cBase = monthlyPayout - cInterest;
                leftBaseByAdvance = leftBaseByAdvance - cBase;
                sumInterestByAdvance = sumInterestByAdvance + cInterest;
                flag = true;
                
                // 假设拿x元做投资的回报金额是:
                paybackByInvest = paybackByInvest + (x+paybackByInvest)*(ratioByInvest/12);
                //cout<<"monthIndex: "<<monthIndex<<", paybackByInvest: "<<paybackByInvest<<endl;
            }
            
            double curInterest = leftBase * (ratio/12);
            double curBase = monthlyPayout - curInterest;
            leftBase = leftBase - curBase;
            sumInterest = sumInterest + curInterest;
            
            if (!flag) {
                sumInterestByAdvance = sumInterest;
                leftBaseByAdvance = leftBase;
            }
            
            
            
            // 作对比
            
            /*
            if ((totalPayout -base - sumInterest - paybackByInvest) > (monthlyPayout*333+x - base - sumInterestByAdvance) && (monthIndex >= indexByAdvance) ) {
                cout<<"提前还款在第"<<monthIndex<<"个月开始奏效"<<endl;
            }*/
            
            if ((sumInterest-paybackByInvest) > sumInterestByAdvance) {
                cout<<"提前还款在第"<<monthIndex<<"个月开始奏效"<<endl;
            }
            
            //cout<<monthlyPayout<<endl;
            //cout<<sumInterest<<endl;
            //cout<<monthIndex<<":"<<leftBaseByAdvance<<":"<<sumInterestByAdvance<<endl;
            //cout<<monthIndex<<":"<<leftBase<<":"<<sumInterest<<endl;
            cout<<"monthIndex: "<<monthIndex<<", sumInterest: "<<sumInterest - paybackByInvest<<", sumInterestByAdvance: "<<sumInterestByAdvance<<endl;
            
            //cout<<"monthIndex: "<<monthIndex<<", paybackByInvest: "<<paybackByInvest<<", leftInterest: "<<totalPayout -base - sumInterest - paybackByInvest<<",leftBase: "<<leftBase<<", leftInterestInAdvance:"<<monthlyPayout*333+x - base - sumInterestByAdvance<<", leftBaseInAdvance: "<<leftBaseByAdvance<<endl;
            
            monthIndex++;
            //cout<<monthIndex<<endl;
            
        }
        
        return 1;
    }
};

结论

似乎只要是投资利率高于还款利率就不亏。