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Copy pathBinaryTreeLevelOrderTraversal.java
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92 lines (71 loc) · 2.69 KB
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// Given a binary tree, return the level order traversal of its nodes' values.
// (ie, from left to right, level by level).
// See: https://leetcode.com/problems/binary-tree-level-order-traversal/
package leetcode.tree;
import static leetcode.util.tree.BinTreeUtil.*;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.LinkedHashMap;
import java.util.LinkedList;
import java.util.List;
import java.util.Map;
import java.util.Queue;
import leetcode.util.tree.TreeNode;
public class BinaryTreeLevelOrderTraversal {
/**
* DFS with preorder traversal implementation.
*/
public List<List<Integer>> levelOrder(TreeNode root) {
Map<Integer, List<Integer>> resMap = new LinkedHashMap<Integer, List<Integer>>();
dfs(root, 0, resMap);
return new ArrayList<>(resMap.values());
}
private void dfs(TreeNode root, int level, Map<Integer, List<Integer>> resultMap) {
if (root == null)
return;
if (!resultMap.containsKey(level)) {
resultMap.put(level, new ArrayList<>());
}
resultMap.get(level).add(root.val);
dfs(root.left, level + 1, resultMap);
dfs(root.right, level + 1, resultMap);
}
/**
* BFS with queue implementation
*/
public List<List<Integer>> levelOrder_bfs(TreeNode root) {
class Pair {
final TreeNode root;
final int level;
Pair(TreeNode root, int level) {
this.root = root;
this.level = level;
}
}
Queue<Pair> q = new LinkedList<>();
q.add(new Pair(root, 0));
Map<Integer, List<Integer>> res = new HashMap<>();
while (!q.isEmpty()) {
Pair currPair = q.poll();
TreeNode currNode = currPair.root;
int currLevel = currPair.level;
if (currNode != null) {
if (!res.containsKey(currLevel)) {
res.put(currLevel, new ArrayList<>());
}
res.get(currLevel).add(currNode.val);
if (currNode.left != null)
q.add(new Pair(currNode.left, currLevel + 1));
if (currNode.right != null)
q.add(new Pair(currNode.right, currLevel + 1));
}
}
return new ArrayList<>(res.values());
}
public static void main(String[] args) {
BinaryTreeLevelOrderTraversal sln = new BinaryTreeLevelOrderTraversal();
TreeNode t1 = initTree(3, 9, 20, null, null, 15, 7);
System.out.println(sln.levelOrder(t1));
System.out.println(sln.levelOrder(null));
}
}