Additive Number solution and explanation

def is_additive_number(num):
    def is_valid(num1, num2, rest):
        if not rest:
            return True
        next_num = str(int(num1) + int(num2))
        if not rest.startswith(next_num):
            return False
        return is_valid(num2, next_num, rest[len(next_num):])

    n = len(num)
    for i in range(1, n//2 + 1):
        for j in range(1, n//2 + 1):
            num1, num2 = num[:i], num[i:i+j]
            if ((num1.startswith("0") and len(num1) > 1) or
                (num2.startswith("0") and len(num2) > 1)):
                continue
            if is_valid(num1, num2, num[i+j:]):
                return True
    return False

#include <stdbool.h>
#include <string.h>

bool is_valid(const char* num, int start, long long num1, long long num2) {
    if (start == strlen(num)) return true;

    long long sum = num1 + num2;
    char sum_str[50];
    sprintf(sum_str, "%lld", sum);
    int len = strlen(sum_str);

    if (strncmp(num + start, sum_str, len) != 0) return false;
    return is_valid(num, start + len, num2, sum);
}

bool isAdditiveNumber(char* num) {
    int n = strlen(num);
    for (int i = 1; i <= n / 2; ++i) {
        for (int j = 1; j <= (n - i) / 2; ++j) {
            if (num[0] == '0' && i > 1) break;
            if (num[i] == '0' && j > 1) break;

            long long num1 = atoll(strndup(num, i));
            long long num2 = atoll(strndup(num + i, j));

            if (is_valid(num, i + j, num1, num2)) return true;
        }
    }
    return false;
}

public class Solution {
    public boolean isAdditiveNumber(String num) {
        int n = num.length();
        for (int i = 1; i <= n / 2; i++) {
            if (num.charAt(0) == '0' && i > 1) break;
            Long num1 = Long.parseLong(num.substring(0, i));
            for (int j = 1; Math.max(i, j) <= n - i - j; j++) {
                if (num.charAt(i) == '0' && j > 1) break;
                Long num2 = Long.parseLong(num.substring(i, i + j));
                if (isValid(i + j, num1, num2, num)) return true;
            }
        }
        return false;
    }

    private boolean isValid(int start, Long num1, Long num2, String num) {
        while (start < num.length()) {
            num2 = num2 + num1;
            num1 = num2 - num1;
            String sumStr = num2.toString();
            if (!num.substring(start).startsWith(sumStr)) return false;
            start += sumStr.length();
        }
        return true;
    }
}

function isAdditiveNumber(num) {
    const n = num.length;
    for (let i = 1; i <= Math.floor(n / 2); i++) {
        for (let j = 1; j <= Math.floor((n - i) / 2); j++) {
            if (num[0] == '0' && i > 1) break;
            if (num[i] == '0' && j > 1) break;

            const num1 = BigInt(num.substring(0, i));
            const num2 = BigInt(num.substring(i, i + j));

            if (isValid(i + j, num1, num2, num)) return true;
        }
    }
    return false;
}

function isValid(start, num1, num2, num) {
    while (start < num.length) {
        num2 = num2 + num1;
        num1 = num2 - num1;
        const sumStr = num2.toString();
        if (!num.substring(start).startsWith(sumStr)) return false;
        start += sumStr.length;
    }
    return true;
}

class Solution {
public:
    bool isAdditiveNumber(string num) {
        int n = num.size();
        for (int i = 1; i < min(n - 1, 19); ++i) {
            for (int j = i + 1; j < min(n, i + 19); ++j) {
                if (i > 1 && num[0] == '0') break;
                if (j - i > 1 && num[i] == '0') continue;
                auto a = stoll(num.substr(0, i));
                auto b = stoll(num.substr(i, j - i));
                if (dfs(a, b, num.substr(j, n - j))) return true;
            }
        }
        return false;
    }

    bool dfs(long long a, long long b, string num) {
        if (num == "") return true;
        if (a + b > 0 && num[0] == '0') return false;
        for (int i = 1; i < min((int) num.size() + 1, 19); ++i)
            if (a + b == stoll(num.substr(0, i)))
                if (dfs(b, a + b, num.substr(i, num.size() - i)))
                    return true;
        return false;
    }
};

func isAdditiveNumber(num string) bool {
	n := len(num)
	var dfs func(a, b int64, num string) bool
	dfs = func(a, b int64, num string) bool {
		if num == "" {
			return true
		}
		if a+b > 0 && num[0] == '0' {
			return false
		}
		for i := 1; i < min(len(num)+1, 19); i++ {
			c, _ := strconv.ParseInt(num[:i], 10, 64)
			if a+b == c {
				if dfs(b, c, num[i:]) {
					return true
				}
			}
		}
		return false
	}
	for i := 1; i < min(n-1, 19); i++ {
		for j := i + 1; j < min(n, i+19); j++ {
			if i > 1 && num[0] == '0' {
				break
			}
			if j-i > 1 && num[i] == '0' {
				continue
			}
			a, _ := strconv.ParseInt(num[:i], 10, 64)
			b, _ := strconv.ParseInt(num[i:j], 10, 64)
			if dfs(a, b, num[j:]) {
				return true
			}
		}
	}
	return false
}