leetcode 306 : Additive Number

1、原题如下

Additive number is a string whose digits can form additive sequence.

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.

Follow up:
How would you handle overflow for very large input integers?

2、解题如下：

class Solution {public:    bool isAdditiveNumber(string num) {        int a=num.size();        if(a<3) return false;        for(int i=1;i<=a-2;i++)        {            string tmp1=num.substr(0,i);//check            if(tmp1.size()>1&&tmp1[0]=='0') break;            long num1=stol(tmp1);//stoi            for(int j=i+1;j<=a-1;j++)//check            {                string tmp2=num.substr(i,j-i);                if(tmp2.size()>1&&tmp2[0]=='0') break;                long num2=stol(tmp2);                if(isAdditiveNumber(num1,num2,j,num))                    return true;            }        }        return false;    }    bool isAdditiveNumber(long num1,long num2,int count,string num)    {        for(int k=count+1;k<=num.size();k++)        {            string tmp3=num.substr(count,k-count);            if(tmp3.size()>1&&tmp3[0]=='0') break;            long num3=stol(tmp3);            if(num3==num1+num2)            {                if(k==num.size())                    return true;                else                    return isAdditiveNumber(num2,num3,k,num);            }        }        return false;    }};

3、总结
本题之所以采用long是因为位数不够，如果long不足够的话，可以尝试long long ，就能解决问题了，如果long long 就需要stoll