二叉树的所有路径（递归，迭代）Java

递归法

class Solution {
    
    public List binaryTreePaths(TreeNode root) {
        List res = new ArrayList<>();
        if (root == null) {
            return res;
        }
        List paths = new ArrayList<>();
        traversal(root, paths, res);
        return res;
    }

    private void traversal(TreeNode root, List paths, List res) {
        paths.add(root.val);
        // 叶子结点
        if (root.left == null && root.right == null) {
            // 输出
            StringBuilder sb = new StringBuilder();
            for (int i = 0; i < paths.size() - 1; i++) {
                sb.append(paths.get(i)).append("->");
            }
            sb.append(paths.get(paths.size() - 1));
            res.add(sb.toString());
            return;
        }
        if (root.left != null) {
            traversal(root.left, paths, res);
            paths.remove(paths.size() - 1);// 回溯
        }
        if (root.right != null) {
            traversal(root.right, paths, res);
            paths.remove(paths.size() - 1);// 回溯
        }
    }
}

迭代法


class Solution {
    public List binaryTreePaths(TreeNode root) {
         List paths = new ArrayList();
        if (root == null) {
            return paths;
        }
        Queue nodeQueue = new linkedList();
        Queue pathQueue = new linkedList();

        nodeQueue.offer(root);
        pathQueue.offer(Integer.toString(root.val));

        while (!nodeQueue.isEmpty()) {
            TreeNode node = nodeQueue.poll(); 
            String path = pathQueue.poll();

            if (node.left == null && node.right == null) {
                paths.add(path);
            } else {
                if (node.left != null) {
                    nodeQueue.offer(node.left);
                    pathQueue.offer(new StringBuffer(path).append("->").append(node.left.val).toString());
                }

                if (node.right != null) {
                    nodeQueue.offer(node.right);
                    pathQueue.offer(new StringBuffer(path).append("->").append(node.right.val).toString());
                }
            }
        }
        return paths;


    }
}