Leetcode——67. Add Binary

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问题

Given two binary strings, return their sum (also a binary string).

For example,
a = “11”
b = “1”
Return “100”.

解答

*class Solution {public:    string addBinary(string a, string b) {        int i=0;        string res="";        int carryout=0;        while(i<a.length()||i<b.length())        {            int alast=a.length()-i-1;            int blast=b.length()-i-1;            if(alast>=0)                carryout=carryout+(a[a.length()-i-1]-'0');            if(blast>=0)                carryout=carryout+(b[b.length()-i-1]-'0');            res=char (carryout%2+'0')+res;            carryout=carryout/2;            i++;        }        if(carryout==1)           res="1"+res;         return res;    }};*

这一题很奇怪的一个地方是：

            int alast=a.length()-i-1;            int blast=b.length()-i-1;            if(alast>=0)                carryout=carryout+(a[a.length()-i-1]-'0');            if(blast>=0)                carryout=carryout+(b[b.length()-i-1]-'0');

这部分代码换成

            if(a.length()-i-1>=0)                carryout=carryout+(a[a.length()-i-1]-'0');            if(b.length()-i-1>=0)                carryout=carryout+(b[b.length()-i-1]-'0');

之后就是运行不对，放在VS2015下跑，
中间打印`

cout<<b.length()-i-1<<endl

得出很大的一个数

但是一旦把

int blast=b.length()-i-1;cout<<blast<<endl

得出的结果是正常的，这是为啥呢？？？？？？？？？？

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