LeetCode(306) Addtive Number解题报告

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?

解题思路:

此题重点在于枚举取出第一个和第二个数，因为第一个数的取值位数不会超过num长度的一半,所以可以让i循环到(num.length + 1) / 2，然后添加辅助函数，利用递归完成整个addtiveNumber的判断，返回结果。

public static boolean isAdditiveNumber(String num) {
String first = null;
String second = null;
for(int i = 1; i < num.length(); i++)
for(int j = i+1; j < num.length(); j++){
first = num.substring(0, i);
second = num.substring(i,j);
if(isValid(num,first,second,j))
return true;
}
return false;

}
public static boolean isValid(String num,String first,String second,int start){

if(num.length() == start)
return true;
long a = Long.parseLong(first);
long b = Long.parseLong(second);
if(!("" + a).equals(first) || !("" + b).equals(second))
return false;

long c = a + b;
String res = "" + c;
if((res.length() + start) > num.length())
return false;
if(num.substring(start,start + res.length()).equals(res)){
return isValid(num,second,res,(start + res.length()));
}
return false;
}