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  • 二叉树遍历(前序、中序、后序)-Java实现

    一、前序遍历

      访问顺序:先根节点,再左子树,最后右子树;上图的访问结果为:GDAFEMHZ。

      1)递归实现

        public void preOrderTraverse1(TreeNode root) {
            if (root != null) {
                System.out.print(root.val + "->");
                preOrderTraverse1(root.left);
                preOrderTraverse1(root.right);
            }
        }

      2)非递归实现

        public void preOrderTraverse2(TreeNode root) {
            Stack<TreeNode> stack = new Stack<>();
            TreeNode node = root;
            while (node != null || !stack.empty()) {
                if (node != null) {
                    System.out.print(node.val + "->");
                    stack.push(node);
                    node = node.left;
                } else {
                    TreeNode tem = stack.pop();
                    node = tem.right;
                }
            }
        }

    二、中序遍历

      访问顺序:先左子树,再根节点,最后右子树;上图的访问结果为:ADEFGHMZ。

      1)递归实现

        public void inOrderTraverse(TreeNode root) {
            if (root != null) {
                inOrderTraverse(root.left);
                System.out.print(root.val + "->");
                inOrderTraverse(root.right);
            }
        }

      2)非递归实现

        public void inOrderTraverse(TreeNode root) {
            Stack<TreeNode> stack = new Stack<>();
            TreeNode node = root;
            while (node != null || !stack.isEmpty()) {
                if (node != null) {
                    stack.push(node);
                    node = node.left;
                } else {
                    TreeNode tem = stack.pop();
                    System.out.print(tem.val + "->");
                    node = tem.right;
                }
            }
        }

    三、后序遍历

      访问顺序:先左子树,再右子树,最后根节点,上图的访问结果为:AEFDHZMG。

      1)递归实现

        public void postOrderTraverse(TreeNode root) {
            if (root != null) {
                postOrderTraverse(root.left);
                postOrderTraverse(root.right);
                System.out.print(root.val + "->");
            }
        }

       2)非递归实现

        public void postOrderTraverse(TreeNode root) {
            TreeNode cur, pre = null;
    
            Stack<TreeNode> stack = new Stack<>();
            stack.push(root);
    
            while (!stack.empty()) {
                cur = stack.peek();
                if ((cur.left == null && cur.right == null) || (pre != null && (pre == cur.left || pre == cur.right))) {
                    System.out.print(cur.val + "->");
                    stack.pop();
                    pre = cur;
                } else {
                    if (cur.right != null)
                        stack.push(cur.right);
                    if (cur.left != null)
                        stack.push(cur.left);
                }
            }
        }

     四、层次遍历

      访问结果:GDMAFHZE。

        public void levelOrderTraverse(TreeNode root) {
            if (root == null) {
                return;
            }
            Queue<TreeNode> queue = new LinkedList<TreeNode>();
            queue.add(root);
    
            while (!queue.isEmpty()) {
                TreeNode node = queue.poll();
                System.out.print(node.val + "->");
    
                if (node.left != null) {
                    queue.add(node.left);
                }
                if (node.right != null) {
                    queue.add(node.right);
                }
            }
        }
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  • 原文地址:https://www.cnblogs.com/zhi-leaf/p/10813048.html
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