逆波兰式计算，将格式化数组转化为逆波兰表达式，a为输入的数组，n为其长度，p为输出逆波兰表达式的目标，m记录逆波兰表达式的长度 -inverse Polish, the array will be formatted into reverse Polish expression, a for the importation of arrays, its length n, p output of the inverse expression Poland, Poland inverse m record the length of expression