class Solution {
public:
    string mergeAlternately(string a, string b) {
        string res;
        int i=0,j=0;
        while(i<a.size()||j<b.size())
        {
            if(i<a.size()) res+=a[i++];//分别判断
            if(j<b.size()) res+=b[j++];
        }
           return res;
    }

};