addStrings.java

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.

//TC: O(n) where n is max length of two arrays, because we loop through them bothß
//SC: O(n) where n is max length between num1 and num2 to store char array
class Solution {
    public String addStrings(String num1, String num2) {
        char[] nums1 = num1.toCharArray();
        char[] nums2 = num2.toCharArray();
        
        StringBuilder sb = new StringBuilder();
        
        int carry = 0; 
        
        int i =nums1.length-1;
        int j =nums2.length-1;
        
        while(i>=0 || j>=0){
            int sum = carry; 
            if(i>=0){
                sum+= nums1[i] - '0';
                i--;
            }
            if(j>=0){
                sum+= nums2[j] -'0';
                j--;
            }
            carry = sum/10; 
            sb.append(sum%10);
        }
        if(carry >0){
            sb.append(carry);
        }
        
        sb.reverse(); 
        return sb.toString();
    }
}