131 - 415 字符串相加

题目

给定两个字符串形式的非负整数 num1 和num2 ，计算它们的和。

注意：

1. num1 和num2 的长度都小于 5100.

2. num1 和num2 都只包含数字 0-9.

3. num1 和num2 都不包含任何前导零。

4. 你不能使用任何內建 BigInteger 库， 也不能直接将输入的字符串转换为整数形式。

解答

不能转成整数，因此要模拟加法。。所以还是要每位数转成数字。。

https://leetcode-cn.com/problems/add-strings/solution/add-strings-shuang-zhi-zhen-fa-by-jyd/

感觉不应该用字符串，每次增加都会新建一个字符串，感觉很浪费内存。

不如每次都塞进数组里面，最后排序并变成字符串输出比较好。

var addStrings = function(num1, num2) {
  let res = [],
    i = num1.length - 1,
    j = num2.length - 1,
    carry = 0
  while (i >= 0 || j >= 0) {
    const n1 = i < 0 ? 0 : parseInt(num1[i])
    const n2 = j < 0 ? 0 : parseInt(num2[j])
    const temp = n1 + n2 + carry
    carry = ~~(temp / 10)
    res.push(temp % 10)
    i--
    j--
  }
  if (carry) {
    res.push(carry)
  }
  return res.reverse().join("")
};

Runtime: 64 ms, faster than 79.98% of JavaScript online submissions for Add Strings.

Memory Usage: 36.4 MB, less than 37.50% of JavaScript online submissions for Add Strings.

class Solution:
    def addStrings(self, num1: str, num2: str) -> str:
        res = ""
        i = len(num1)-1
        j = len(num2)-1
        carry = 0
        while i >= 0 or j >= 0:
            n1 = 0 if i < 0 else int(num1[i])
            n2 = 0 if j < 0 else int(num2[j])
            temp = n1+n2+carry
            carry = temp//10
            res = str(temp % 10)+res
            i -= 1
            j -= 1
        return "1"+res if carry else res

Runtime: 52 ms, faster than 60.04% of Python3 online submissions for Add Strings.

Memory Usage: 13.7 MB, less than 5.55% of Python3 online submissions for Add Strings.

class Solution:
    def addStrings(self, num1: str, num2: str) -> str:
        res = []
        i = len(num1)-1
        j = len(num2)-1
        carry = 0
        while i >= 0 or j >= 0:
            n1 = 0 if i < 0 else int(num1[i])
            n2 = 0 if j < 0 else int(num2[j])
            temp = n1+n2+carry
            carry = temp//10
            res.append(temp % 10)
            i -= 1
            j -= 1
        if carry:
            res.append(carry)
        return "".join(str(i) for i in res[::-1])

Runtime: 40 ms, faster than 92.18% of Python3 online submissions for Add Strings.

Memory Usage: 13.9 MB, less than 5.55% of Python3 online submissions for Add Strings.

好吧。。只是运行速度快了一点，并没有减少多少内存

func addStrings(num1 string, num2 string) string {
    i := len(num1) - 1
    j := len(num2) - 1
    var res []int
    carry := 0
    for i >= 0 || j >= 0 {
        var n1, n2 int
        if i < 0 {
            n1 = 0
        } else {
            n1, _ = strconv.Atoi(string(num1[i]))
        }
        if j < 0 {
            n2 = 0
        } else {
            n2, _ = strconv.Atoi(string(num2[j]))
        }
        temp := n1 + n2 + carry
        carry = temp / 10
        res = append(res, temp%10)
        i--
        j--
    }
    if carry == 1 {
        res = append(res, carry)
    }
    var ans string
    for i := len(res) - 1; i >= 0; i-- {
        ans += strconv.Itoa(res[i])
    }
    return ans
}

Runtime: 4 ms, faster than 53.77% of Go online submissions for Add Strings.

Memory Usage: 6.8 MB, less than 60.00% of Go online submissions for Add Strings.