前序遍歷

public List<Integer> preorderTraversal(TreeNode root) {
    List<Integer> res = new ArrayList<>();
    Deque<TreeNode> stack = new ArrayDeque<>();
    if(root == null)  return res;
    stack.push(root);
    while(!stack.isEmpty()) {
        TreeNode curr = stack.pop();
        res.add(curr.val);
        if(curr.right != null) stack.push(curr.right);
        if(curr.left != null) stack.push(curr.left);
    }
    return res;
}
 

中序遍歷

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

後序遍歷

public List<Integer> postorderTraversal(TreeNode root) {
    List<Integer> res = new ArrayList<>();
    if (root == null) return res;
    Deque<TreeNode> stack = new ArrayDeque<>();
    TreeNode prev = null;
    while (!stack.isEmpty() || root!=null) {
        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;
}