---
title: 'LeetCode 67. Add Binary'
disqus: hackmd
---

# LeetCode 67. Add Binary

## Description

Given two binary strings a and b, return their sum as a binary string.

## Example

Input: a = "11", b = "1"
Output: "100"

Input: a = "1010", b = "1011"
Output: "10101"

## Constraints

1 <= a.length, b.length <= 10^4^
a and b consist only of '0' or '1' characters.
Each string does not contain leading zeros except for the zero itself.

## Answer

此題可先抓出最大長度，ans開最大長度的memory。都先從字串尾開始檢查，其中，-'0'為字元轉整數，+ '0'為整數轉字元。如全部計算完但仍需進位，就直接給定為'1'，如不需進位，就將指標移往下一項即可跳過空項即可。

//2022_03_30
char * addBinary(char * a, char * b){
    int i = 0, size = 0, la = strlen(a), lb = strlen(b), pls = 0;
    char *opa = a + la - 1, *opb = b + lb - 1;
    size = la > lb ? la+1 : lb+1;
    char *ans = (char*)malloc(sizeof(char)*(size+1));
    ans[size--] = '\0';
    while(opa >= a || opb >= b){
        if(opa >= a && opb >= b){
            ans[size] = ((*opa - '0') + (*opb - '0') + pls) % 2 + '0';
            pls = ((*opa - '0') + (*opb - '0') + pls) / 2;
        }
        else if(opa >= a){
            ans[size] = ((*opa - '0') + pls) % 2 + '0';
            pls = ((*opa - '0') + pls) / 2;
        }
        else {
            ans[size] = ((*opb - '0') + pls) % 2 + '0';
            pls = ((*opb - '0') + pls) / 2;
        }
        opa--;
        opb--;
        size--;
    }
    if(size == 0){
        if(pls){ans[size] = '1';}
        else{ans++;}
    }
    return ans;
}

## Link
https://leetcode.com/problems/add-binary/
###### tags: `Leetcode`