二叉树的中序遍历
给定一个二叉树,返回它的中序 遍历。
示例:
输入: [1,null,2,3]
1
\
2
/
3
输出: [1,3,2]
非递归(思路更清晰):
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
class Solution {
public List<Integer> inorderTraversal(TreeNode root) {
ArrayList<Integer> list = new ArrayList<>();
Stack<TreeNode> stack = new Stack<>();
if(root == null) return list;
TreeNode cur = root;
//将currentNode移动到最左边
while(cur != null){
stack.push(cur);
cur = cur.left;
}
while(!stack.isEmpty()){
//根结点的访问条件是只要左子树访问完毕,就可以访问根结点(左节点为空或者左节点为上次访问节点)
cur = stack.pop();
list.add(cur.val);
//切换到右子树
cur = cur.right;
//访问右子树的最左边
while(cur != null){
stack.push(cur);
cur = cur.left;
}
}
return list;
}
}
非递归:
\/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
class Solution {
public List<Integer> inorderTraversal(TreeNode root) {
ArrayList<Integer> list = new ArrayList<>();
Stack<TreeNode> stack = new Stack<>();
if(root == null) return list;
TreeNode cur = root;
while(cur != null || !stack.isEmpty()){
if(cur != null){
stack.push(cur);
cur = cur.left;
}else{
cur = stack.pop();
list.add(cur.val);
cur =cur.right;
}
}
return list;
}
}
递归:
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
class Solution {
ArrayList<Integer> list = new ArrayList<>();
public List<Integer> inorderTraversal(TreeNode root) {
if(root == null) return new ArrayList<>();
inorderTraversal(root.left);
list.add(root.val);
inorderTraversal(root.right);
return list;
}
}
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