[LeetCode] 60. Permutation Sequence

有点想不明白，为什么要做 k = k - 1 ┑(￣▽ ￣)┍

	class Solution {
	public String getPermutation(int n, int k) {
	String chooseFrom = "123456789";
	int[] factorial = new int[n + 1];
	factorial[0] = 1;
	factorial[1] = 1;
	for (int i = 2; i <= n; i++) {
	factorial[i] = factorial[i - 1] * i;
	}
	k = k - 1; // Why?
	StringBuilder sb = new StringBuilder();
	for (int i = n; i >= 1; i--) {
	int x = k / factorial[i - 1];
	k = k % factorial[i - 1];
	sb.append(chooseFrom.charAt(x));
	chooseFrom = chooseFrom.substring(0, x) + chooseFrom.substring(x + 1, chooseFrom.length());
	}
	return sb.toString();
	}
	}

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