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LeetCode107.java

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package problems;

import java.util.ArrayList;

import java.util.LinkedList;

import java.util.List;

import java.util.Queue;

/**

* BFS 做法

*/

public class LeetCode107 {

public List<List<Integer>> levelOrderBottom(TreeNode root) {

List<List<Integer>> levelOrder = new LinkedList<List<Integer>>();

if (root == null) {

return levelOrder;

}

Queue<TreeNode> queue = new LinkedList<TreeNode>();

queue.offer(root);

while (!queue.isEmpty()) {

List<Integer> level = new ArrayList<Integer>();

int size = queue.size();

for (int i = 0; i < size; i++) {

TreeNode node = queue.poll();

level.add(node.val);

TreeNode left = node.left, right = node.right;

if (left != null) {

queue.offer(left);

}

if (right != null) {

queue.offer(right);

}

}

levelOrder.add(0, level);

}

return levelOrder;

}

}

/**

* DFS 做法

*/

class LeetCode107_1 {

public List<List<Integer>> levelOrderBottom(TreeNode root) {

List<List<Integer>> res = new LinkedList<>();

dfs(root, 0, res);

return res;

}

public void dfs(TreeNode node, int depth, List<List<Integer>> res) {

if(node == null) {

return;

}

if(depth == res.size()) {

res.add(0, new ArrayList<>());

}

res.get(res.size() - depth - 1).add(node.val);

dfs(node.left, depth + 1, res);

dfs(node.right, depth + 1, res);

}

}