文章目录
491.递增子序列
文字讲解 :递增子序列
视频讲解 :递增子序列
**状态:这题看了文字讲解才AC,掌握了如何在回溯里通过Set集合来对同层节点去重
思路:
代码:
java
class Solution {
List<List<Integer>> result = new ArrayList<>();
LinkedList<Integer> tempList = new LinkedList<>();
public List<List<Integer>> findSubsequences(int[] nums) {
backTracking(nums, 0);
return result;
}
//本题的关键在于,同层不能有重复元素,当前层的节点不能大于上一层的值
public void backTracking(int[] nums, int startIndex) {
if (startIndex>=nums.length) {
return;
}
//借助set集合去重
HashSet hs = new HashSet();
for (int i = startIndex; i < nums.length; i++) {
if ((!tempList.isEmpty() && tempList.get(tempList.size()-1) > nums[i]) || hs.contains(nums[i])) {
continue;
}
hs.add(nums[i]);
tempList.offer(nums[i]);
if (tempList.size()>1) {
result.add(new ArrayList<>(tempList));
}
backTracking(nums, i+1);
tempList.pollLast();
}
}
}
46.全排列
文字讲解 :全排列
视频讲解 :全排列
状态:做完组合类的题,这题好简单
思路:
代码:
java
class Solution {
List<List<Integer>> result = new ArrayList<>();
LinkedList<Integer> tempList = new LinkedList<>();
boolean[] usedArr;
public List<List<Integer>> permute(int[] nums) {
this.usedArr = new boolean[nums.length];
for (int i = 0; i < this.usedArr.length; i++) {
this.usedArr[i] = false;
}
backTracking(nums);
return result;
}
public void backTracking(int[] nums) {
if (tempList.size()==nums.length) {
//收集
result.add(new ArrayList<>(tempList));
return;
}
for (int i = 0; i < nums.length; i++) {
if (usedArr[i]) {
continue;
}
usedArr[i]=true;
tempList.offer(nums[i]);
backTracking(nums);
tempList.pollLast();
usedArr[i]=false;
}
}
}
47.全排列II
文字讲解 :全排列II
视频讲解 :全排列
状态:将前两题的思路整合,这题ok
思路:
代码:
java
class Solution {
List<List<Integer>> result = new ArrayList<>();
LinkedList<Integer> tempList = new LinkedList<>();
boolean[] used;
public List<List<Integer>> permuteUnique(int[] nums) {
Arrays.sort(nums);
this.used = new boolean[nums.length];
for (int i = 0; i < used.length; i++) {
used[i] = false;
}
backTracking(nums);
return result;
}
public void backTracking(int[] nums) {
if (tempList.size()==nums.length) {
result.add(new ArrayList<>(tempList));
return;
}
HashSet<Integer> hs = new HashSet();
for (int i = 0; i < nums.length; i++) {
if (used[i] || hs.contains(nums[i])) {
continue;
}
hs.add(nums[i]);
used[i] = true;
tempList.offer(nums[i]);
backTracking(nums);
tempList.pollLast();
used[i] = false;
}
}
}