LeetCode0094-二叉树的遍历

迭代算法实现二叉树的遍历

//实现二叉树的前序、中序、后序遍历

import java.util.ArrayList;
import java.util.List;
import java.util.Stack;

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;
      }
  }

 

class ColorNode{
    TreeNode node;
    String color;
    public ColorNode(TreeNode node,String color){
        this.node=node;
        this.color=color;
    }
}
//递归算法

public class Num094_binaryTree {

    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        if(root == null) return res;
        inorder(root,res);
        return res;
    }
    //        中序： 左根右
    public void inorder(TreeNode node ,List<Integer> res){
        if(node ==null) return;
        inorder(node.left,res);
        res.add(node.val);
        inorder(node.right,res);
    }
    //        前序：根左右
    public void beforeorder(TreeNode node,List<Integer> res){
        if(node ==null) return;
        res.add(node.val);
        beforeorder(node.left,res);
        beforeorder(node.right,res);
    }
    //        后续：左右根
    public void afterorder(TreeNode node,List<Integer> res){
        if(node ==null) return;
        beforeorder(node.left,res);
        beforeorder(node.right,res);
        res.add(node.val);
    }

    public List<Integer> inorderTraversal1(TreeNode root){
        if (root ==null ) return new ArrayList<Integer>();

        List<Integer> res = new ArrayList<>();
        Stack<ColorNode> stack= new Stack<>();
        stack.push(new ColorNode(root,"white"));

        while(!stack.empty()){
            ColorNode cn = stack.pop();

            if(cn.color.equals("white")){
                if(cn.node.right !=null) stack.push(new ColorNode(cn.node.right,"white"));
                stack.push(new ColorNode(cn.node,"gray"));
                if(cn.node.left !=null) stack.push(new ColorNode(cn.node.left,"white"));
            }else{
                res.add(cn.node.val);
            }
        }
        return res;

    }

}

 

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