二叉树前、中、后、层次、遍历的非递归法

class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        if(root == null) return res;
        Stack<TreeNode> st = new Stack<>();
        st.push(root);
        while(!st.empty()){
            TreeNode node = st.pop();
            res.add(node.val);
            if(node.right != null) st.push(node.right);
            if(node.left != null) st.push(node.left);
        }
        return res;
    }
}

class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        if(root == null) return res;
        Stack<TreeNode> st = new Stack<>();
        TreeNode cur = root;
        while(!st.empty() || cur != null) {
            while(cur != null){
                st.push(cur);
                cur = cur.left;
            }
            cur = st.pop();
            res.add(cur.val);
            cur = cur.right;
        }
        return res;
    }
}

class Solution {
    public List<Integer> postorderTraversal(TreeNode root) {
        List<Integer> res = new LinkedList<>();
        if(root == null) return res;
        Stack<TreeNode> st = new Stack<>();
        st.push(root);
        while(!st.isEmpty()){
            TreeNode node = st.pop();
            res.add(0,node.val);
            if (node.left != null) st.push(node.left);
            if (node.right != null) st.push(node.right);
        }
        return res;
    }
}

class Solution {
    public List<List<Integer>> levelOrder(TreeNode root) {
        List<List<Integer>> res = new ArrayList<>();
        Queue<TreeNode> queue = new LinkedList<>();
        if(root == null) return res;
        queue.add(root);
        while(!queue.isEmpty()){
            List<Integer> list = new LinkedList<>();
            int size = queue.size();
            while(size-- > 0){
                TreeNode node = queue.poll();
                list.add(node.val);
                if(node.left!=null) queue.add(node.left);
                if(node.right!=null) queue.add(node.right);
            }
            res.add(list);
        }
        return res;
    }
}