94_二叉树的中序遍历

94_二叉树的中序遍历

package 二叉树.BT;

import java.util.ArrayList;
import java.util.Deque;
import java.util.LinkedList;
import java.util.List;


public class _94_二叉树的中序遍历 {
    public class TreeNode {
        int val;
        TreeNode left;
        TreeNode right;

        TreeNode() {
        }

        TreeNode(int val) {
            this.val = val;
        }

        TreeNode(int val, TreeNode left, TreeNode right) {
            this.val = val;
            this.left = left;
            this.right = right;
        }
    }
    // 递归实现的
    List<Integer> list = new ArrayList<>();
    public List<Integer> inorderTraversal(TreeNode root) {
        if(root == null) return list;
        inorderTraversal(root.left);
        //拿到当前结点
        list.add(root.val);
        inorderTraversal(root.right);
        return list;
    }

    // 迭代
    public List<Integer> inorderTraversal2(TreeNode root) {
        List<Integer> list2 = new ArrayList<>();
        if (root == null)
            return list2;
        Deque<TreeNode> stack = new LinkedList<>();
        TreeNode node = root;
        while(node != null || !stack.isEmpty() ) {
            while(node != null) {
                stack.push(node);
                node = node.left;
            }
            node = stack.pop();
            list2.add(node.val);    //已经拿到当前结点了    
            node = node.right;
        }
        return list2;
    }

}