[leetcode] Permutations

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Given a collection of numbers, return all possible permutations.

For example,
[1,2,3]have the following permutations:
[1,2,3],[1,3,2],[2,1,3],[2,3,1],[3,1,2], and[3,2,1].

https://oj.leetcode.com/problems/permutations/

思路：生成permutation，这题假设没有重复元素，递归求解，每一次在cur位置填元素，添之前判断钙元素是否已经用过，没用过继续下去，指导cur==n。

public class Solution {
	public ArrayList<ArrayList<Integer>> permute(int[] num) {
		if (num == null)
			return null;
		ArrayList<ArrayList<Integer>> res = new ArrayList<ArrayList<Integer>>();
		if (num.length == 0)
			return res;
		int[] permSeq = new int[num.length];
		perm(num.length, 0, num, permSeq, res);
		return res;
	}

	private void perm(int n, int cur, int[] num, int[] perm,
			ArrayList<ArrayList<Integer>> res) {
		if (cur == n) {
			ArrayList<Integer> tmp = new ArrayList<Integer>();
			for (int i = 0; i < perm.length; i++) {
				tmp.add(perm[i]);
			}
			res.add(tmp);
		} else {
			int i;
			for (i = 0; i < num.length; i++) {
				int j;
				boolean ok = true;
				for (j = cur - 1; j >= 0; j--) {
					if (perm[j] == num[i])
						ok = false;
				}
				if (ok) {
					perm[cur] = num[i];
					perm(n, cur + 1, num, perm, res);
				}
			}
		}

	}

	public static void main(String[] args) {
		System.out.println(new Solution().permute(new int[] { 1, 2, 3,4 }));
	}
}

[leetcode] Permutations,布布扣,bubuko.com

[leetcode] Permutations

标签：class   code   java   http   tar   com   

原文地址：http://www.cnblogs.com/jdflyfly/p/3810755.html