Robinhood Stock Market Return of Investment Calculation Algorithm

You are given integers n, e, o, t. You have n dollars in principal that you invested in the stock market. Given the stock market alternates between first returning e and then o percent interest per year, return how many years it would take to reach at leastt dollars.

Example 1
Input
n = 100
e = 20
o = 10
t = 130
Output
2
Explanation
In the first year we earn interest of 20% and end up with $120.
In the second year we earn interest of 10% and end up with $132, which is above the target $130.

Algorithm of Robinhood Stock ROI

We update the current principal based on the year count. One pitfall is use 32-bit integer to accomodate for the current value which may overflow. You can use double or int64 to avoid the investment value overflow.

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int robinhoodStockReturnOfInvestment(int n, int e, int o, int t) {
    int res = 0;
    double nn = n;
    while (nn < t) {
        if (res & 1) {
            nn = nn * ((100.0 + o) / 100.0);
        } else {
            nn = nn * ((100.0 + e) / 100.0);
        }
        res ++;
    }
    return res;
}

int robinhoodStockReturnOfInvestment(int n, int e, int o, int t) {
    int res = 0;
    double nn = n;
    while (nn < t) {
        if (res & 1) {
            nn = nn * ((100.0 + o) / 100.0);
        } else {
            nn = nn * ((100.0 + e) / 100.0);
        }
        res ++;
    }
    return res;
}

In Python, we don’t need to worry about the integer overflow.

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class Solution:
    def robinhoodStockReturnOfInvestment(self, n, e, o, t):
        res = 0
        while n < t:
            if res & 1:
                n = n * (100 + o) / 100
            else:
                n = n * (100 + e) / 100
            res += 1
        return res

class Solution:
    def robinhoodStockReturnOfInvestment(self, n, e, o, t):
        res = 0
        while n < t:
            if res & 1:
                n = n * (100 + o) / 100
            else:
                n = n * (100 + e) / 100
            res += 1
        return res

–EOF (The Ultimate Computing & Technology Blog) —