var num = prompt('请输入一个三位数');
        var a, b, c;
        if (num / 1000 >= 1) {
            alert('请输入一个三位数');
        } else {
            if (isNaN(a) || isNaN(b) || isNaN(c)) {
                alert('输入有误');
            } else {
                a = parseInt(num / 100);
                b = parseInt(num % 100 / 10);
                c = parseInt(num % 100 % 10);
                alert('该数的百位为' + a + '十位为' + b + '个位为' + c);
            }
        }