The arguments are passed as strings.
The numbers may be way very large
Answer should be returned as a string
The returned "number" should not start with zeros e.g. 0123 is invalid

function multiply(a, b) {
        // 通常的算法操作
        /*
        例如 123 * 456
             123
           * 456
             ---
             738
            615
           492
           ----
           56088           
         */
        // 为了方便计算，先把数字反转，进位就不朝前了，而是朝后  到最后再反转回来
        var aa = a.split('').reverse(); // 
        var bb = b.split('').reverse();

        var stack = [];

        for (var i = 0; i < aa.length; i++) {
            for (var j = 0; j < bb.length; j++) {
                var m = aa[i] * bb[j];
                stack[i + j] = (stack[i + j]) ? stack[i + j] + m : m;
            }
        }
        // 组合成一维数组 给下面进位操作

        for (var i = 0; i < stack.length; i++) {
            var num = stack[i] % 10; 
            var move = Math.floor(stack[i] / 10);
            stack[i] = num;
             
            // 大于10 采取进位操作
            if (stack[i + 1])
                stack[i + 1] += move;
            else if (move != 0)
                stack[i + 1] = move;
        }


        return stack.reverse().join('').replace(/^(0(?!$))+/, ""); // 替换字符串前面的0 
    }