十进制转任意进制，任意进制转的十进制

#include <algorithm>
#include <string>

std::string decimalToK(int num, int k) {
    if (k < 2 || k > 16) return "Invalid"; // 进制范围校验[7](@ref)
    if (num == 0) return "0"; // 特殊值处理[3](@ref)
    
    const char digits[] = "0123456789ABCDEF"; // 字符映射表[7](@ref)
    bool isNegative = num < 0;
    num = abs(num);
    
    std::string result;
    while (num > 0) {
        int remainder = num % k;
        result += digits[remainder]; // 直接查表优化[7](@ref)
        num /= k;
    }
    
    if (isNegative) result += '-';
    std::reverse(result.begin(), result.end()); // 倒序修正[2](@ref)
    return result;
}


#include <cctype>

int base_to_dec(const std::string& num, int base) {
    if (base < 2 || base > 36) return -1;
    int result = 0;
    for (char c : num) {
        int value = isdigit(c) ? c - '0' : toupper(c) - 'A' + 10;
        if (value >= base) return -1; // 非法字符检测
        result = result * base + value;
    }
    return result;
}