二叉树非递归遍历

代码

public class BinaryTreeTraversal {

    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> result = new ArrayList<>();
        Deque<TreeNode> stack = new ArrayDeque<>();
        TreeNode p = root;
        while(!stack.isEmpty() || p != null) {
            if(p != null) {
                stack.push(p);
                result.add(p.val);  // Add before going to children
                p = p.left;
            } else {
                TreeNode node = stack.pop();
                p = node.right;
            }
        }
        return result;
    }

    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> result = new ArrayList<>();
        Deque<TreeNode> stack = new ArrayDeque<>();
        TreeNode p = root;
        while(!stack.isEmpty() || p != null) {
            if(p != null) {
                stack.push(p);
                p = p.left;
            } else {
                TreeNode node = stack.pop();
                result.add(node.val);  // Add after all left children
                p = node.right;
            }
        }
        return result;
    }

    public List<Integer> postorderTraversal(TreeNode root) {
        LinkedList<Integer> result = new LinkedList<>();
        Deque<TreeNode> stack = new ArrayDeque<>();
        TreeNode p = root;
        while(!stack.isEmpty() || p != null) {
            if(p != null) {
                stack.push(p);
                result.addFirst(p.val);  // Reverse the process of preorder
                p = p.right;             // Reverse the process of preorder
            } else {
                TreeNode node = stack.pop();
                p = node.left;           // Reverse the process of preorder
            }
        }
        return result;
    }

}

参考来源

• https://leetcode.com/problems/binary-tree-postorder-traversal/discuss/45551/Preorder-Inorder-and-Postorder-Iteratively-Summarization