自己的解法：
2x2 种情况判断

    function solution(urlStr) {const pre = urlStr.split(",")[0];const after = urlStr.split(",")[1];if (pre.endsWith("/")) {if (after.startsWith("/")) {return pre + after.slice(1);} else {return after + pre;}} else {if (after.startsWith("/")) {return pre + after;} else {return pre + "/" + after;}}}console.log(solution("/abc/,/bcd"));//in: /acm,/bb     /abc/,/bcd//out: /acm/bb     /abc/bcd