二叉树之前序、中序、后序、层序遍历

二叉树的遍历

二叉树的表示

//Java
public class TreeNode{
    int val;
    Node left,right;
    public Node(int val){
        this.val=val;
    }
}

前序遍历

递归方法

    public static void preOrder1(TreeNode root){
        if(root == null)
            return;
        System.out.println(root.val);
        preOrder1(root.left);
        preOrder1(root.right);
    }

非递归方法

    public static void preOrder2(TreeNode root){
        Stack<TreeNode>  stack = new Stack<>();
        TreeNode node = root;
        while(node != null || !stack.empty()){
            if(node != null){
                System.out.println(node.val);
                stack.push(node);
                node = node.left;
            }else{
                node = stack.pop();
                node = node.right;
            }
        }
    }

中序遍历

递归方法

    public static void inOrder1(TreeNode root){
        if(root == null)
            return;
        inOrder1(root.left);
        System.out.println(root.val);
        inOrder1(root.right);
    }

非递归方法

    //非递归方法
    public static void inOrder2(TreeNode root){
        Stack<TreeNode> stack = new Stack<TreeNode>();
        TreeNode node= root;
        while(node != null || !stack.empty()){
            if(node != null){
                stack.push(node);
                node = node.left;
            }else {
                node = stack.pop();
                System.out.println(node.val);
                node = node.right;
            }
        }
    }

后序遍历

递归方法

    public static void postOrder1(TreeNode root){
        if(root == null)
            return;
        postOrder1(root.left);
        postOrder1(root.right);
        System.out.println(root.val);
    }

非递归方法

    //非递归方法
    public static void postOrder2(TreeNode root){
        Stack<TreeNode> stack  = new Stack<TreeNode>();
        TreeNode node = root;
        TreeNode lastNode = root;
        while(node != null || !stack.empty()){
            while(node != null){
                stack.push(node);
                node = node.left;
            }

            node = stack.peek();

            if(node.right == null || node.right == lastNode){
                System.out.println(node.val);
                stack.pop();
                lastNode = node;
                node = null;
            }else {
                node = node.right;
            }


        }

层序遍历

 public static void LevelorderTraversal ( TreeNode root )
 {
     LinkedList<TreeNode> Q = new LinkedList<TreeNode>();
     TreeNode node = root;

     if ( node == null ) return; /* 若是空树则直接返回 */
     Q.offer(node);

     while ( !Q.isEmpty() ) {
         node = Q.poll();
         System.out.println( node.val ); /* 访问取出队列的结点 */
         if ( node.left != null )   Q.offer(node.left);
         if ( node.right != null )   Q.offer(node.right);
     }
}