树的遍历——迭代

/*中序遍历*/

class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> list=new ArrayList<>();
        Deque<TreeNode> stack=new LinkedList<>();
        while(root!=null || !stack.isEmpty()){
            while(root!=null){
                stack.push(root);
                root=root.left;
            }
            root=stack.pop();
            list.add(root.val);
            root=root.right;
        }
        return list;
    }
}

/*前序遍历*/

class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> list=new ArrayList<>();
        Deque<TreeNode> stack=new LinkedList<>();
        while(!stack.isEmpty() || root!=null){
            while(root!=null){
                list.add(root.val);
                stack.push(root);
                root=root.left;
            }
            root=stack.pop();
            root=root.right;
        }
        return list;
    }
}

/*后序遍历*/

class Solution {
    public List<Integer> postorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<Integer>();
        if (root == null) {
            return res;
        }
        Deque<TreeNode> stack = new LinkedList<TreeNode>();
        TreeNode prev = null;
        while (root != null || !stack.isEmpty()) {
            while (root != null) {
                stack.push(root);
                root = root.left;
            }
            root = stack.pop();
            if (root.right == null || root.right == prev) {
                res.add(root.val);
                prev = root;
                root = null;
            } else {
                stack.push(root);
                root = root.right;
            }
        }
        return res;
    }
}