關於二叉樹的幾種遍歷

import java.util.*;


/*二叉樹遍歷測試，有一點難，前序與後序可以對照着看*/
public class Solution3 {
    /*二叉樹前序遍歷    根左右*/
    public static ArrayList<Integer> testPre(TreeNode root){
        ArrayList<Integer> list=new ArrayList<Integer>();
        Stack<TreeNode> stack=new Stack<TreeNode>();
        if(root==null){
            return list;
        }
        stack.push(root);
        while (!stack.empty()){
            TreeNode node=stack.pop();
            list.add(node.val);
            if(node.right!=null){
                stack.push(node.right);
            }
            if(node.left!=null){
                stack.push(node.left);
            }
        }
        return list;
    }

 /*後序遍歷   左右根   可以換算成  根右左，利用stack先進後出變成左右根。*/
    public static ArrayList<Integer> testLast(TreeNode root){
        ArrayList<Integer> list=new ArrayList<Integer>();
        Stack<TreeNode> stack=new Stack<TreeNode>();
        Stack<Integer> stack1=new Stack<Integer>();
        if(root==null){
            return list;
        }
        stack.push(root);
        while (!stack.empty()){
            TreeNode node=stack.pop();
            stack1.push(node.val);
            if(node.left!=null){
                stack.push(node.left);
            }
            if(node.right!=null){
                stack.push(node.right);
            }
        }
        while (!stack1.empty()){
            int a=stack1.pop();
            list.add(a);
        }
        return list;
        }

/*中序二叉樹遍歷     左根右*/
public static ArrayList<Integer> testMid(TreeNode root){
    ArrayList<Integer> list=new ArrayList<Integer>();
    Stack<TreeNode> stack=new Stack<TreeNode>();
    if(root==null){
        return list;
    }
    //邊界條件，全部遍歷完才能結束。
    while (!stack.empty()||root!=null){
        if (root != null) {
            stack.push(root);
            root = root.left;
        }else {
            TreeNode node = stack.pop();
            list.add(node.val);
            root = node.right;
        }
    }
    return list;
}

    /*層級遍歷*/
    public static ArrayList<Integer> testFloor(TreeNode root) {
        ArrayList<Integer> list = new ArrayList<Integer>();
        Queue<TreeNode> queue=new ArrayDeque<TreeNode>();
        if(root==null){
            return list;
        }
        queue.offer(root);
        while (!queue.isEmpty()){
            TreeNode node=queue.poll();
            list.add(node.val);
        if (node.left!=null){
            queue.offer(node.left);
        }
        if (node.right!=null){
            queue.offer(node.right);
        }
        }
        return list;
    }



public static void main(String[] args) {
    TreeNode tree=new TreeNode(1);
    tree.left=new TreeNode(2);
    tree.right=new TreeNode(3);
    tree.left.left=new TreeNode(4);
    tree.left.right=new TreeNode(5);
    tree.right.left=new TreeNode(6);
    tree.right.right=new TreeNode(7);
    //遍歷測試
    ArrayList<Integer> l=testPre(tree);
    ArrayList<Integer> l1=testMid(tree);
    ArrayList<Integer> l2=testLast(tree);
    ArrayList<Integer> l3=testFloor(tree);
    for (Integer i : l1) {
        System.out.println(i);
    }

}
}

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