Permutation Sequence
25% Accepted
Given n and k, return the k-th permutation sequence.
Have you met this question in a real interview? Yes
Example
For n = 3, all permutations are listed as follows:
"123"
"132"
"213"
"231"
"312"
"321"
If k = 4, the fourth permutation is "231"
Note
n will be between 1 and 9 inclusive.
Challenge
O(n*k) in time complexity is easy, can you do it in O(n^2) or less?
Tags Expand
Permutation Array
思路
同样先通过举例来获得更好的理解。以n = 4,k = 9为例:
1234
1243
1324
1342
1423
1432
2134
2143
2314 <= k = 9
2341
2413
2431
3124
3142
3214
3241
3412
3421
4123
4132
4213
4231
4312
4321
最高位可以取{1, 2, 3, 4},而每个数重复3! = 6次。所以第k=9个permutation的s[0]为{1, 2, 3, 4}中的第9/6+1 = 2个数字s[0] = 2。
而对于以2开头的6个数字而言,k = 9是其中的第k' = 9%(3!) = 3个。而剩下的数字{1, 3, 4}的重复周期为2! = 2次。所以s[1]为{1, 3, 4}中的第k'/(2!)+1 = 2个,即s[1] = 3。
对于以23开头的2个数字而言,k = 9是其中的第k'' = k'%(2!) = 1个。剩下的数字{1, 4}的重复周期为1! = 1次。所以s[2] = 1.
对于以231开头的一个数字而言,k = 9是其中的第k''' = k''/(1!)+1 = 1个。s[3] = 4
public class Solution {
public String getPermutation(int n, int k) {
// initialize all numbers
ArrayList<Integer> numberList = new ArrayList<Integer>();
for (int i = 1; i <= n; i++) {
numberList.add(i);
}
// change k to be index
k--;
// set factorial of n
int mod = 1;
for (int i = 1; i <= n; i++) {
mod = mod * i;
}
String result = "";
// find sequence
for (int i = 0; i < n; i++) {
mod = mod / (n - i);
// find the right number(curIndex) of
int curIndex = k / mod;
// update k
k = k % mod;
// get number according to curIndex
result += numberList.get(curIndex);
// remove from list
numberList.remove(curIndex);
}
return result.toString();
}
}