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.
public class Solution { public boolean isAdditiveNumber(String num) { if(num == null || num.length() == 0) return false; int n = num.length(); for(int i = 1; i <= n/2; i++){ // len of x1 for(int j = 1; Math.max(i, j) + i + j <= n; j++) { // len of x2 if(isValid(i, j, num)) return true; } } return false; } public boolean isValid(int i, int j, String num){ if(i > 1 && num.charAt(0) == "0") return false; if(j > 1 && num.charAt(i) == "0") return false; String sum; Long x1 = Long.parseLong(num.substring(0, i)); Long x2 = Long.parseLong(num.substring(i, i+j)); for(int start = i + j; start < num.length(); start += sum.length()){ x2 = x1 + x2; // sum of x1 and x2, and became new x2 x1 = x2 - x1; // last x2 is new x1 sum = x2.toString(); if(!num.startsWith(sum, start)) return false; } return true; } }
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