编程导航算法村第七关 |二叉树的遍历

编程导航算法村第七关 | 二叉树的遍历

前序遍历（递归）

 public List<Integer> preorderTraversal(TreeNode root) {
        ArrayList<Integer> result = new ArrayList<Integer>();
        preorder(root, result);
        return result;
    }

    public void preorder(TreeNode root, List<Integer> result) {
        if (root == null) {
            return;
        }
        result.add(root.val);
        preorder(root.left, result);
        preorder(root.right, result);
    }

前序遍历（迭代）

• 先迭代到树的最底层，左左端的元素，然后弹出栈，访问他的右节点

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

        Stack<TreeNode> stack = new Stack<TreeNode>();

        TreeNode node = root;
        while (!stack.isEmpty() || node != null) {
            while (node!= null) {
                result.add(node.val);
                stack.push(node);
                node = node.left;
            }
            node = stack.pop();
            node = node.right;
        }
        return result;
    }

中序遍历（迭代）

 public List<Integer> inorderTraversal(TreeNode root) {
        ArrayList<Integer> result = new ArrayList<>();
        if (root == null) {
            return result;
        }
        Stack<TreeNode> stack = new Stack<TreeNode>();

        TreeNode node = root;
        while (!stack.isEmpty() || node != null) {
            while (node != null) {
                stack.push(node);
                node = node.left;
            }
            node = stack.pop();
            result.add(node.val);
            node = node.right;
        }
        return result;
    }

后续遍历（反转法）

• 后续遍历相当于在前序遍历的基础上，先访问右节点再访问左节点，最后翻转

 public List<Integer> postorderTraversal(TreeNode root) {
        ArrayList<Integer> result = new ArrayList<>();
        if (root == null) {
            return result;
        }
        Stack<TreeNode> stack = new Stack<TreeNode>();

        TreeNode node = root;
        while (!stack.isEmpty() || node != null) {
            while (node != null) {
                stack.push(node);
                result.add(node.val);
                node = node.right;
            }
            node = stack.pop();
            node = node.left;
        }
        Collections.reverse(result);
        return result;
    }