二叉树递归、迭代、层序遍历

二叉树递归、迭代、层序遍历总结

递归遍历

• 前序遍历

//前序遍历顺序：中-左-右，入栈顺序：中-右-左
class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<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);
    }
}

• 中序遍历

// 中序遍历顺序: 左-中-右 入栈顺序： 左-右
class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        inorder(root, res);
        return res;
    }

    void inorder(TreeNode root, List<Integer> list) {
        if (root == null) {
            return;
        }
        inorder(root.left, list);
        list.add(root.val);             // 注意这一句
        inorder(root.right, list);
    }
}

• 后序遍历

// 后序遍历顺序 左-右-中 入栈顺序：中-左-右 出栈顺序：中-右-左， 最后翻转结果
class Solution {
 public List<Integer> postorderTraversal(TreeNode root) {
     List<Integer> res = new ArrayList<>();
     postorder(root, res);
     return res;
 }

 void postorder(TreeNode root, List<Integer> list) {
     if (root == null) {
         return;
     }
     postorder(root.left, list);
     postorder(root.right, list);
     list.add(root.val);             // 注意这一句
 }
}

迭代遍历——应用栈结构

 * 前序遍历

// 前序遍历顺序：中-左-右，入栈顺序：中-右-左
class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> result = new ArrayList<>();
        if (root == null){
            return result;
        }
        Stack<TreeNode> stack = new Stack<>();
        stack.push(root);
        while (!stack.isEmpty()){
            TreeNode node = stack.pop();
            result.add(node.val);
            if (node.right != null){
                stack.push(node.right);
            }
            if (node.left != null){
                stack.push(node.left);
            }
        }
        return result;
    }
}

 * 中序遍历

// 中序遍历顺序: 左-中-右 入栈顺序： 左-右
class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> result = new ArrayList<>();
        if (root == null){
            return result;
        }
        Stack<TreeNode> stack = new Stack<>();
        TreeNode cur = root;
        while (cur != null || !stack.isEmpty()){
           if (cur != null){
               stack.push(cur);
               cur = cur.left;
           }else{
               cur = stack.pop();
               result.add(cur.val);
               cur = cur.right;
           }
        }
        return result;
    }
}

 * 后序遍历

// 后序遍历顺序 左-右-中 入栈顺序：中-左-右 出栈顺序：中-右-左， 最后翻转结果
class Solution {
    public List<Integer> postorderTraversal(TreeNode root) {
        List<Integer> result = new ArrayList<>();
        if (root == null){
            return result;
        }
        Stack<TreeNode> stack = new Stack<>();
        stack.push(root);
        while (!stack.isEmpty()){
            TreeNode node = stack.pop();
            result.add(node.val);
            if (node.left != null){
                stack.push(node.left);
            }
            if (node.right != null){
                stack.push(node.right);
            }
        }
        Collections.reverse(result);
        return result;
    }
}

层序遍历——应用队列结构

//记录每一层的数量，一层一层的遍历
public void checkFun02(TreeNode node) {
        if (node == null) return;
        Queue<TreeNode> que = new LinkedList<TreeNode>();
        que.offer(node);

        while (!que.isEmpty()) {
            List<Integer> itemList = new ArrayList<Integer>();
            int len = que.size();

            while (len > 0) {
                TreeNode tmpNode = que.poll();
                itemList.add(tmpNode.val);

                if (tmpNode.left != null) que.offer(tmpNode.left);
                if (tmpNode.right != null) que.offer(tmpNode.right);
                len--;
            }

            resList.add(itemList);
        }

    }