The set [1,2,3,…,n]
contains a total of n! unique permutations.
By listing and labeling all of the permutations in order,
We get the following sequence (ie, for n = 3):
"123"
"132"
"213"
"231"
"312"
"321"
Given n and k th, return the kth permutation sequence.
Note: Given n will be between 1 and 9 inclusive.
我刚开始尝试着把,所有的全排列都列出来,然后,找到对应位置的内容,但是这样效率太低,直接超时了,上网差了一下,和数学有关系。。。这题打算先放一放,放一下别人的代码吧。
public String getPermutation(int n, int k) {
char[] nums = new char[]{'1','2','3','4','5','6','7','8','9'};
String tmp = "";
for(int i=0;i<n;i++) {
tmp += nums[i];
}
StringBuffer s = new StringBuffer(tmp);
String r = "";
while(k>0&&!s.toString().equals("")) {
// 计算 (n-1)的排列个数cnt
int cnt = 1, i = s.length()-1;
while(i > 1) {
cnt*=i;
i-=1;
}
int pos = (k-1)/cnt;
r += s.charAt(pos);
s = s.deleteCharAt(pos);
k -= pos * cnt;
}
return r;
}
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