Given two non-negative integers num1 and num2 represented as string, return the sum of num1 and num2.
Note:
The length of both num1 and num2 is < 5100.
Both num1 and num2 contains only digits 0-9.
Both num1 and num2 does not contain any leading zero.
You must not use any built-in BigInteger library or convert the inputs to integer directly.
Solution：
思路：
Time Complexity: O(N) Space Complexity: O(1)
Solution Code：
public class Solution {
public String addStrings(String num1, String num2) {
StringBuilder sb = new StringBuilder();
int carry = 0;
for(int i = num1.length() - 1, j = num2.length() - 1; i >= 0 || j >= 0 || carry == 1; i--, j--){
int x = i < 0 ? 0 : num1.charAt(i) - '0';
int y = j < 0 ? 0 : num2.charAt(j) - '0';
sb.append((x + y + carry) % 10);
carry = (x + y + carry) / 10;
}
return sb.reverse().toString();
}
}
