solution.ts

function convertToBase7 (num: number): string {
    if (num === 0) {
        return '0';
    }
    let prefix = '';
    if (num < 0) {
        prefix = '-';
        num = -num;
    }
    const stack:number[] = [];
    while (num > 0) {
        const mod = num % 7;
        const rest = (num - mod) / 7;
        stack.push(mod);
        num = rest;
    }
    return `${prefix}${stack.reverse().join('')}`;
}