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PermutationSequence.java
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67 lines (61 loc) · 1.6 KB
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package algorithm.lc;
/**
* 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, return the kth permutation sequence.
*
* Note: Given n will be between 1 and 9 inclusive.
*
*/
// O(n) space, O(kn) time
public class PermutationSequence {
public class Solution {
public String getPermutation(int n, int k) {
// Start typing your Java solution below
// DO NOT write main() function
int[] num = new int[n];
for (int i = 0; i < n; ++i) {
num[i] = i + 1;
}
for (int i = 1; i < k; ++i) {
next(num);
}
StringBuilder sb = new StringBuilder();
for (int i : num) {
sb.append(i);
}
return sb.toString();
}
private void next(int[] num) {
if (num.length == 1) {
return;
}
int lastSmaller = num.length - 2;
while (num[lastSmaller] >= num[lastSmaller + 1]) {
--lastSmaller;
if (lastSmaller < 0) {
return;
}
}
int lastBigger = num.length - 1;
while (num[lastBigger] <= num[lastSmaller]) {
--lastBigger;
}
swap(num, lastSmaller++, lastBigger);
int last = num.length - 1;
while (lastSmaller < last) {
swap(num, lastSmaller++, last--);
}
}
private void swap(int[] num, int first, int second) {
int tmp = num[first];
num[first] = num[second];
num[second] = tmp;
}
}
}