leetcode--Permutations

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].

 

public class Solution {
    /**
	 * dynamic programming method.
	 * the permutation contains num[0] is one.
	 * For num[1], there are two place to insert it and get two lists <num[1], num[0]>, <num[0], num[1]>. Using
	 * this idea, if we have insert num[i - 1] and get their permutaton, now we insert num[i] into each list.
	 * There are (i + 1) position (=length of list + 1) to place num[i].
	 * @param num --Integer array.
	 * @return List<List<Integer>> --the collection of all permutation of num.
	 * @author Averill Zheng
	 * @version 2014-06-05
	 * @since JDK 1.7
	 */
	public List<List<Integer>> permute(int[] num) {
		List<List<Integer> > result = new ArrayList<List<Integer>>();
		int length = num.length;
		if(length > 0){
			List<Integer> startList = new ArrayList<Integer>();
			startList.add(num[0]);
			result.add(startList);
			int size = 1;
			for(int i = 1; i < length; ++i){
				List<List<Integer>> tempResult = new ArrayList<List<Integer> >();
				for(int j = 0; j < size; ++j){
					List<Integer> aList = result.get(j);
					for(int k = 0; k < i + 1; ++k){
						List<Integer> newList = new ArrayList<Integer>();
						newList.addAll(aList);
						newList.add(k, num[i]);
						tempResult.add(newList);
					}	
				}
				size *= (i + 1);
				result = tempResult;
			}
		}
		return result;
    }
}