Given a binary tree, return the zigzag level order traversal of its nodes’ values. (ie, from left to right, then right to left for the next level and alternate between).
For example: Given binary tree [3,9,20,null,null,15,7],
3
/ \
9 20
/ \
15 7
return its zigzag level order traversal as:
[
[3],
[20,9],
[15,7]
]
Solution:
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode() {}
* TreeNode(int val) { this.val = val; }
* TreeNode(int val, TreeNode left, TreeNode right) {
* this.val = val;
* this.left = left;
* this.right = right;
* }
* }
*/
class Solution {
public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
List<List<Integer>> result = new ArrayList<>();
dfs(root, result, 0);
return result;
}
void dfs(TreeNode node, List<List<Integer>> result, int level) {
if (node == null) {
return;
}
checkSize(result, level);
if (level % 2 == 1) {
result.get(level).add(0, node.val);
} else {
result.get(level).add(node.val);
}
dfs(node.left, result, level + 1);
dfs(node.right, result, level + 1);
}
void checkSize(List<List<Integer>> result, int level) {
// 0 [ [] ]
// 1 [[][]]
// 2 [ [][] ]
if (result.size() <= level) {
result.add(new ArrayList<>());
}
}
}
Solution 2021-11-18 DFS, DFS + Stack, BFS
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode() {}
* TreeNode(int val) { this.val = val; }
* TreeNode(int val, TreeNode left, TreeNode right) {
* this.val = val;
* this.left = left;
* this.right = right;
* }
* }
*/
class Solution {
public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
List<List<Integer>> res = new ArrayList<>();
Queue<TreeNode> q = new ArrayDeque<>();
if (root != null) {
q.add(root);
}
int level = 0;
while (q.size() > 0) {
int size = q.size();
List<Integer> levelList = new ArrayList();
for (int i = 0; i < size; i++) {
TreeNode node = q.poll();
if (level % 2 == 0) {
levelList.add(node.val);
} else {
levelList.add(0, node.val);
}
if (node.left != null) {
q.add(node.left);
}
if (node.right != null) {
q.add(node.right);
}
}
res.add(levelList);
level++;
}
return res;
}
public List<List<Integer>> zigzagLevelOrderDfsStack(TreeNode root) {
List<List<Integer>> res = new ArrayList<>();
Stack<Pair<TreeNode, Integer>> stack = new Stack<>();
if (root != null) {
stack.push(new Pair<>(root, 0));
}
while (stack.size() > 0) {
Pair<TreeNode, Integer> curr = stack.pop();
TreeNode node = curr.getKey();
Integer level = curr.getValue();
if (res.size() == level) {
res.add(new ArrayList<>());
}
if (level % 2 == 0) {
res.get(level).add(node.val);
} else {
res.get(level).add(0, node.val);
}
if (node.right != null) {
stack.push(new Pair<>(node.right, level + 1));
}
if (node.left != null) {
stack.push(new Pair<>(node.left, level + 1));
}
}
return res;
}
public List<List<Integer>> zigzagLevelOrderDfsRecursive(TreeNode root) {
List<List<Integer>> res = new ArrayList<>();
dfs(root, 0, res);
return res;
}
void dfs(TreeNode node, int level, List<List<Integer>> res) {
if (node == null) return;
if (res.size() == level) {
res.add(new ArrayList<>());
}
if (level % 2 == 0) {
res.get(level).add(node.val);
} else {
res.get(level).add(0, node.val);
}
dfs(node.left, level + 1, res);
dfs(node.right, level + 1, res);
}
}