kth permutation

Author: okmd

backtracking/lb-19-kth-permutation-of-sequence.cpp

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+#include<iostream>
+#include<vector>
+using namespace std;
+// Brute force is to generate all possible permutations; sort then find value at k-1th element;
+string getPermutation(int n, int k) {
+    vector<int> nums;
+    string ans;
+    int fact = 1;
+    for (int i = 1; i <= n; i++) {
+        nums.push_back(i); // create set for 1,2,3....n
+        fact *= i; // n!
+    }
+    k--; // 0-based indexing
+    while (nums.size()) {
+        fact /= nums.size(); // reduce factorial as one element is reuced;
+        int loc = k / fact;
+        ans += to_string(nums[loc]); // 1|{234} -> 3!=6 same for 2,3,4
+        nums.erase(nums.begin() + loc); // erase the picked element
+        k %= fact; // k lies in this group of (n-1)! so find new index of k
+    }
+    return ans;
+}
+int main() {
+    int n = 7;
+    int k = 1000;
+    cout << getPermutation(n, k) << endl;
+    return 0;
+}