[LeetCode]Permutation sequence

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数学找规律

public String getPermutation(int n, int k) {  
    if(n<=0)  
        return "";  
    k--;  
    StringBuilder res = new StringBuilder();  
    int factorial = 1;  
    ArrayList<Integer> nums = new ArrayList<Integer>();  
    //initialize factorial
    for(int i=2;i<n;i++)  
        factorial *= i;  
    //initialize nums
    for(int i=1;i<=n;i++)  
        nums.add(i);   
    int round = n-1;  
    while(round>=0)  
    {  
        int index = k/factorial;  
        k %= factorial;  
        res.append(nums.get(index));  
        nums.remove(index);  
        if(round>0)  
            factorial /= round;  
        round--;  
    }  
    return res.toString();  
}