Similar Problems:

A valid additive sequence should contain at least three numbers. Except for the first two numbers, each subsequent number in the sequence must be the sum of the preceding two.

For example:
“112358” is an additive number because the digits can form an additive sequence: 1, 1, 2, 3, 5, 8.

```1 + 1 = 2, 1 + 2 = 3, 2 + 3 = 5, 3 + 5 = 8
```

“199100199” is also an additive number, the additive sequence is: 1, 99, 100, 199.

```1 + 99 = 100, 99 + 100 = 199
```

Note: Numbers in the additive sequence cannot have leading zeros, so sequence 1, 2, 03 or 1, 02, 3 is invalid.

Given a string containing only digits ‘0’-‘9’, write a function to determine if it’s an additive number.

How would you handle overflow for very large input integers?

Github: code.dennyzhang.com

Credits To: leetcode.com

Leave me comments, if you have better ways to solve.

```// Blog link: https://code.dennyzhang.com/additive-number
// Basic Ideas: Find all possible first two numbers.
// The number might be overflow.
// We may not be able to convert string to int, add them, then convert back to string.
//
// Complexity: Time O(n*n), Space O(n)
import "strconv"

func isAdditiveNumber2(num1 string, num2 string, num string) bool {
if num1[0] == '0' && num1 != "0" { return false }
if num2[0] == '0' && num2 != "0" { return false }
if len(num) < len(num3) {
return false
} else {
if len(num) == len(num3) {
return num == num3
} else {
if num[0:len(num3)] != num3 {
return false
} else {
}
}
}
}

func addTwoStringNumber(num1 string, num2 string) string {
i, j, carry := len(num1)-1, len(num2)-1, 0
res := ""
for i>=0 && j>=0 {
v1, _ := strconv.Atoi(string(num1[i]))
v2, _ := strconv.Atoi(string(num2[j]))
v := carry + v1 + v2
if v >= 10 {
carry = 1
v -= 10
} else {
carry = 0
}
res = fmt.Sprintf("%d", v) + res
i, j = i-1, j-1
}

num := ""
if i>=0 {
num = num1[0:i+1]
}

if j>=0 {
num = num2[0:j+1]
}

if num == "" {
if carry == 1 {
res = "1" + res
}
} else {
if (carry == 0) {
res = num + res
} else {
for i := len(num)-1; i>=0; i-- {
v, _ := strconv.Atoi(string(num[i]))
v += carry
if v>= 10 {
carry = 1
v -= 10
} else {
carry = 0
}
res = fmt.Sprintf("%d", v) + res
}
if carry == 1 {
res = "1" + res
}
}
}
return res
}

// first number: num[0:i]
// second number: num[i:j]
for i := 1; i<len(num)-1; i++ {
for j := i+1; j<len(num); j++ {