Thinking process I - traversal

visit every node

public class Solution {
    /**
     * @param root the root of the binary tree
     * @return all root-to-leaf paths
     */
    public List<String> binaryTreePaths(TreeNode root) {
        List<String> result = new ArrayList<>();
        if (root == null) return result;

        dfsHelper(result, root, String.valueOf(root.val));

        return result;
    }

    private void dfsHelper(List<String> result, TreeNode root, String path) {

        if (root.left == null && root.right == null) {//leaf
            result.add(path);
            return;
        }

        if (root.left != null) {
            dfsHelper(result, root.left, path + "->" + root.left.val);
        }

        if (root.right != null) {
            dfsHelper(result, root.right, path + "->" + root.right.val);
        }

    }
}

Thinking process II - Divide and Conquer

add current node with what is returned from left subtree

then add current node with what is returned from right subtree

Mistakes

don't forget the leaf node

public class Solution {
    /**
     * @param root the root of the binary tree
     * @return all root-to-leaf paths
     */
    public List<String> binaryTreePaths(TreeNode root) {
        List<String> result = new ArrayList<>();

        if (root == null) return result;

        List<String> leftPaths = binaryTreePaths(root.left);
        List<String> rightPaths = binaryTreePaths(root.right);

        for (String path : leftPaths) {
            result.add(root.val + "->" + path);
        }

        for (String path : rightPaths) {
            result.add(root.val + "->" + path);
        }

        if (result.size() == 0) {
            result.add(String.valueOf(root.val));
        }

        return result;
    }
}