Project Ruler 算法练习之 Truncate Prime

问题描述：

The number 3797 has an interesting property. Being prime itself, it is possible to continuously remove digits from left to right, and remain prime at each stage: 3797, 797, 97, and 7. Similarly we can work from right to left: 3797, 379, 37, and 3.

Find the sum of the only eleven primes that are both truncatable from left to right and right to left.

NOTE: 2, 3, 5, and 7 are not considered to be truncatable primes.

1. 把数字从前向后依次截取

2.把数字从后向前依次截取

3.如果都是素数，返回true

实现：

(function(){

//truncate prime

var isTp = function(n){

var strN = n.toString();

for(var d = 0 ;d < strN.length; d++){
var s = strN.substr(0,d);
var s1 = strN.substr(strN.length-1-d,d+1);

var ns = parseInt(s);
var ns1 = parseInt(s1);
for(var i = 2 ;i < ns; i++){
if(ns%i == 0 ) {return false;}
}

for(var i = 2 ;i < ns1; i++){
if(ns1%i == 0 ) {return false;}
}

}


return true;
}

// find out the element that have 11 primes included all truncated  numbers 
for(var i = 10; i< 999999; i++){
if(isTp(i)) {console.log("===============" + i+ "=================");}
}

})();