给定一个二叉树,返回该二叉树由底层到顶层的层序遍历,(从左向右,从叶子节点到根节点,一层一层的遍历)
例如:
给定的二叉树是{3,9,20,#,#,15,7},
3 / \ 9 20 / \ 15 7该二叉树由底层到顶层层序遍历的结果是
[[15,7],[9,20],[3]]
/** * Definition for binary tree * public class TreeNode { * int val; * TreeNode left; * TreeNode right; * TreeNode(int x) { val = x; } * } */ import java.util.*; public class Solution { public ArrayList<ArrayList<Integer>> levelOrderBottom(TreeNode root) { ArrayList<ArrayList<Integer>> List1=new ArrayList<ArrayList<Integer>>(); if(root==null) return List1; Queue<TreeNode> queue=new LinkedList<TreeNode>(); Stack<ArrayList<Integer>> stack=new Stack<ArrayList<Integer>>(); queue.add(root); while(!queue.isEmpty()){ ArrayList<Integer> list1=new ArrayList<Integer>(); int count=queue.size(); while(count>0){ TreeNode temp=queue.poll(); list1.add(temp.val); if(temp.left!=null) queue.add(temp.left); if(temp.right!=null) queue.add(temp.right); count--; } stack.add(list1); } while(!stack.empty()){ List1.add(stack.pop()); } return List1; } }
import java.util.*; public class Solution { public ArrayList<ArrayList<Integer>> levelOrderBottom(TreeNode root) { ArrayList<ArrayList<Integer>> lists = new ArrayList<>(); if(root == null) return lists; Queue<TreeNode> queue = new LinkedList<>(); queue.offer(root); while(!queue.isEmpty()) { int size = queue.size(); ArrayList<Integer> list = new ArrayList<>(); for(int i = 0; i < size; i++) { TreeNode node = queue.poll(); if(node.left != null) queue.offer(node.left); if(node.right != null) queue.offer(node.right); list.add(node.val); } lists.add(list); } Collections.reverse(lists); return lists; } }
巧妙利用队列的入队和出队的顺序
import java.util.*;
public class Solution {
public ArrayList<ArrayList<Integer>> levelOrderBottom(TreeNode root) {
ArrayList<ArrayList<Integer>> result = new ArrayList<>();
if (root == null)
return result;
Queue<TreeNode> queue = new LinkedList<>();
queue.add(root);
while (!queue.isEmpty()) {
ArrayList<Integer> list = new ArrayList<>();
int size = queue.size();
for (int i = 0; i < size; i++) {
TreeNode node = queue.poll();
list.add(node.val);
if (node.left != null)
queue.add(node.left);
if (node.right != null)
queue.add(node.right);
}
result.add(0, list);
}
return result;
}
}
/** * Definition for binary tree * public class TreeNode { * int val; * TreeNode left; * TreeNode right; * TreeNode(int x) { val = x; } * } */ import java.util.*; public class Solution { public ArrayList<ArrayList<Integer>> levelOrderBottom(TreeNode root) { ArrayList<ArrayList<Integer>> list = new ArrayList<ArrayList<Integer>>(); if (root == null) return list; LinkedList<TreeNode> queue = new LinkedList<TreeNode>(); queue.offer(root); int size = 0; TreeNode temp = null; while (!queue.isEmpty()) { size = queue.size(); ArrayList<Integer> tempList = new ArrayList<Integer>(); for (int i = 0; i < size; ++i) { temp = queue.poll(); tempList.add(temp.val); if (temp.left != null) queue.offer(temp.left); if (temp.right != null) queue.offer(temp.right); } list.add(tempList); } for (int i = 0; i < list.size() / 2; ++i) { ArrayList<Integer> tempList = list.get(i); list.set(i, list.get(list.size() - 1 - i)); list.set(list.size() - 1 - i, tempList); } return list; } }
import java.util.*; public class Solution { public ArrayList<ArrayList<Integer>> levelOrderBottom(TreeNode root) { ArrayList<ArrayList<Integer>> res = new ArrayList<ArrayList<Integer>>(); if(root == null) return res; ArrayList<Integer> list = new ArrayList<Integer>(); Queue<TreeNode> q = new LinkedList<TreeNode>(); Stack<TreeNode> s = new Stack<TreeNode>(); q.add(root); q.add(null); while(!q.isEmpty()) { TreeNode r = q.remove(); if(r != null) { s.push(r); if(r.right != null) q.add(r.right); if(r.left != null) q.add(r.left); } else { if(!q.isEmpty()) { s.push(null); q.add(null); } } } while(!s.isEmpty()) { TreeNode r = s.pop(); if(r != null) list.add(r.val); else { res.add(list); list = new ArrayList<Integer>(); } } if(list.size() > 0) res.add(list); return res; } }
import java.util.ArrayList;
import java.util.Collections;
public class Solution {
public ArrayList<ArrayList<Integer>> levelOrderBottom(TreeNode root) {
ArrayList<ArrayList<Integer>> result = new ArrayList<ArrayList<Integer>>();// 用来返回结果集合
ArrayList<TreeNode> list = new ArrayList<TreeNode>();
ArrayList<Integer> temp = new ArrayList<Integer>();
if(root == null)
return result;
list.add(root);
while(!list.isEmpty()){
int count = list.size();
for(int i=0; i< count;i++){
TreeNode node = list.remove(0);
temp.add(node.val);
if(node.left != null){
list.add(node.left);
}
if(node.right != null){
list.add(node.right);
}
if(i == count -1){
result.add(new ArrayList<Integer>(temp));
temp.clear();
}
}
}
Collections.reverse(result);
return result;
}
}
public class BinaryTreeLevelOrderTraversalII {
public ArrayList<ArrayList<Integer>> levelOrderBottom(TreeNode root) {
ArrayList<ArrayList<Integer>> result = new ArrayList<>();
if (root == null) return result;
Queue<TreeNode> oldQueue = new LinkedList<>();
Queue<TreeNode> newQueue = new LinkedList<>();
oldQueue.add(root);
ArrayList<Integer> tempList = new ArrayList<>();
while (!oldQueue.isEmpty() || !newQueue.isEmpty()) {
while (!oldQueue.isEmpty()) {
TreeNode node = oldQueue.peek();
tempList.add(node.val);
if (node.left != null) newQueue.add(node.left);
if (node.right != null) newQueue.add(node.right);
oldQueue.remove(oldQueue.peek());
}
result.add(0, tempList);
tempList = new ArrayList<>();
oldQueue.addAll(newQueue);
newQueue.clear();
}
return result;
}
}
public class Solution { public List<List<Integer>> levelOrderBottom(TreeNode root) { Queue<TreeNode> queue = new LinkedList<TreeNode>(); List<List<Integer>> wrapList = new LinkedList<List<Integer>>(); if(root == null) return wrapList; queue.offer(root); while(!queue.isEmpty()){ int levelNum = queue.size(); List<Integer> subList = new LinkedList<Integer>(); for(int i=0; i<levelNum; i++) { if(queue.peek().left != null) queue.offer(queue.peek().left); if(queue.peek().right != null) queue.offer(queue.peek().right); subList.add(queue.poll().val); } wrapList.add(0, subList); } return wrapList; } }
import java.util.*; public class Solution { public ArrayList<ArrayList<Integer>> levelOrderBottom(TreeNode root) { ArrayList<ArrayList<Integer>> res = new ArrayList<>(); if(root == null) return res; Queue<TreeNode> queue = new LinkedList<>(); queue.add(root); while ( ! queue.isEmpty()) { ArrayList<Integer> list = new ArrayList<>(); int size = queue.size(); for (int i = 0; i < size; i ++ ) { TreeNode temp = queue.poll(); list.add(temp.val); if(temp.left != null) queue.add(temp.left); if(temp.right != null) queue.add(temp.right); } res.add(0,list); } return res; } }