二叉树的中序遍历
Tags: LeetCode, 树, 递归 date: Oct 1, 2020
思路
递归算法
想清楚递归边界是什么，在什么时候开始回溯
非递归算法
大佬提供的颜色算法，可以很好的解决二叉树的遍历问题
代码实现
递归代码
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import java.util.*;
/*
* public class TreeNode {
* int val = 0;
* TreeNode left = null;
* TreeNode right = null;
* }
*/
public class Solution {
/**
*
* @param root TreeNode类
* @return int整型ArrayList
*/
public ArrayList<Integer> inorderTraversal (TreeNode root) {
// write code here
ArrayList<Integer> res = new ArrayList<>();
if(root == null)
return res;
else
inorder(root, res);
return res;
}
private void inorder(TreeNode node, ArrayList<Integer> res){
if(node != null){
inorder(node.left,res);
res.add(node.val);
inorder(node.right,res);
}
}
}
非递归代码
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import java.util.*;
/*
* public class TreeNode {
* int val = 0;
* TreeNode left = null;
* TreeNode right = null;
* }
*/
public class Solution {
/**
*
* @param root TreeNode类
* @return int整型ArrayList
*/
public ArrayList<Integer> inorderTraversal (TreeNode root) {
// write code here
Stack<TreeNode> stack = new Stack<TreeNode>();
ArrayList<Integer> res = new ArrayList<Integer>();
HashMap<TreeNode, Integer> map = new HashMap<TreeNode,Integer>();
if(root == null)
return res;
stack.push(root);
map.put(root,1);
while(!stack.isEmpty()){
TreeNode node = stack.pop();
if(map.get(node) == 1){
if(node.right != null){
stack.push(node.right);
map.put(node.right,1);
}
stack.push(node);
map.put(node,2);
if(node.left != null){
stack.push(node.left);
map.put(node.left, 1);
}
}
else
res.add(node.val);
}
return res;
}
}