leetcode 306 Addictive Number
z
 2019-04-09 10:36:27  

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.

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

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

Example 1:

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Input: "112358"
Output: true 
Explanation: 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

Example 2:

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Input: "199100199"
Output: true 
Explanation: The additive sequence is: 1, 99, 100, 199. 
             1 + 99 = 100, 99 + 100 = 199

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

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import java.util.ArrayList;

public class AdditiveNumber {
  
  public boolean isAdditiveNumber(String num) {
    int n = num.length();
    for (int i=1; i<n; i++){
      for (int j=i+1; j<n; j++) {
        long a = parse(num.substring(0, i));
        long b = parse(num.substring(i, j));
        if (a == -1 || b == -1) continue;
        if (helper(num.substring(j), a, b)) {
          return true;
        }
      }
    }
    return false;
  }

  public boolean helper(String s, long a, long b){
    for (int i=1; i<=s.length(); i++){
      long c = parse(s.substring(0, i));
      if (c == -1) continue;
      if (c == a + b && helper(s.substring(i), b, c))
        return true;
    }
    return false;

  }

  public long parse(String s){
    if (!s.equals("0") && s.startsWith("0")) return -1;
    long res = 0;
    try{
      res = Long.parseLong(s);
    }
    catch (Exception e){
      res = -1;
    }
    return res;
  }
}