LeetCode 1768. 交替合并字符串

class Solution {
public:
    string mergeAlternately(string word1, string word2) {
        int n = word1.size(), m = word2.size();
        int i = 0, j = 0;
        string res;
        while(i < n || j < m){
            if(i < n){
                res.push_back(word1[i]);
                ++i;
            }
            if(j < m){
                res.push_back(word2[j]);
                ++j;
            }
        }
        return res;
    }
};