46. Permutations
Tags: ‘Backtracking’
Given a collection of distinct integers, return all possible permutations.
Example:
Input: [1,2,3] Output: [ [1,2,3], [1,3,2], [2,1,3], [2,3,1], [3,1,2], [3,2,1] ]
Solution
经典的Backtrack问题，参考 https://www.v2ex.com/t/633719#reply0
方法1：模板 92%,里面有些方法效率很低，比如LinkedList contains() ,
class Solution {
public List<List<Integer>> permute(int[] nums) {
List<List<Integer>> result = new LinkedList<>();
backtrack(nums, new LinkedList<Integer>(), result);
return result;
}
private void backtrack(int[] nums, LinkedList<Integer> track, List<List<Integer>> result) {
// termination
if (track.size() == nums.length ) {
result.add(new LinkedList(track));
return;
}
for (int i = 0; i < nums.length; i++) {
// 剪枝
if (track.contains(nums[i])) {
continue;
}
track.add(nums[i]); // 做选择
backtrack(nums, track, result); // 进入下一层决策树
track.removeLast(); // 取消选择
}
}
}