LC 0415 [E] Add Strings - ALawliet/algorithms GitHub Wiki

• use ord('digit') - ord('0') to convert str to int
• use 2 pointers or pop off a stack to add from the ends while nums1 or nums2
• start with 0 and compute (top + bot + carry) value (% 10) before carry (// 10)
• add leftover carry at the end if any for last digit
• reverse result at end [::-1] or deque.appendleft() to get correct order

T: O(max(n1+n2))
S: O(max(n1+n2))

why - ord('0')?

>>> ( ord('9') + ord('5') )
110

>>> ord('0')
48
>>> ( ord('9')-ord('0') + ord('5')-ord('0') )
14

---

class Solution:
    def addStrings(self, num1: str, num2: str) -> str:
        res = deque()

        t = list(num1)
        b = list(num2)
        
        carry = 0
        
        while t or b:
            top = bot = 0
            if t:
                top = ord(t.pop()) - ord('0')
            if b:
                bot = ord(b.pop()) - ord('0')
                
            carry, drop = divmod(top + bot + carry, 10)
            # drop = (top + bot + carry) % 10 # must come first
            # carry = (top + bot + carry) // 10
            
            res.appendleft(str(drop))
            
        if carry:
            res.appendleft(str(carry))
            
        return ''.join(res)

class Solution:
    def addStrings(self, num1: str, num2: str) -> str:
        res = deque()
        carry = 0
        t = len(num1)-1
        b = len(num2)-1
        
        while t >= 0 or b >= 0:
            top = bot = 0
            if t >= 0:
                top = ord(num1[t]) - ord('0')
                t -= 1
            if b >= 0:
                bot = ord(num2[b]) - ord('0')
                b -= 1
            
            carry, drop = divmod(top + bot + carry, 10)
            
            res.appendleft(drop)
            
        if carry: res.appendleft(carry)
            
        return ''.join([str(d) for d in res])