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Permutation Sequence

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Description

https://leetcode.com/problems/permutation-sequence/description/

Solution

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class Solution {
public:
    string getPermutation(int n, int k) {
        vector<int> total = this->sumTotalList(n);
        vector<int> remains_digits;
        string ret(n, 'a');
        for (int i = 1; i <= n; i++){
            remains_digits.push_back(i);
        }
        
        int index = n - 1;
        int remain = 0;
        int count = n;
        
        while (index >= 0){
            int target_index = count * (k - 1) / total[index];
            int target = remains_digits[target_index];
            remains_digits.erase(remains_digits.begin() + target_index);
            ret[n - 1 - index] = '0' + target;
            k -= total[index - 1] * target_index;
            index -= 1;
            count -= 1;
        }
        return ret;
    }
    
    vector<int> sumTotalList(int n) {
        vector<int> ret(1, 1);
        if (n == 0 || n == 1) return ret;
        int index = 2;
        int sum = 1;
        while (index <= n){
            sum *= index;
            ret.push_back(sum);
            index += 1;
        }
        return ret;
    }
};