leetcode257_二叉树所有路径

class Solution {
    public List<String> binaryTreePaths(TreeNode root) {
        List<String> result = new ArrayList();
        if(root==null) return result;
        List<Integer> paths = new ArrayList();
        dfs(root, paths, result);
        return result;
    }
    private void dfs(TreeNode root, List<Integer> paths, List<String> result) {
        paths.add(root.val);
        if(root.left == null && root.right == null) {
            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));
            result.add(sb.toString());
        }
        if(root.left != null) {
            dfs(root.left, paths, result);
            paths.remove(paths.size()-1);
        }
        if(root.right != null) {
            dfs(root.right, paths, result);
            paths.remove(paths.size()-1);
        }
        return;
    }
}

这是一道经典的树上带回溯的题目，可以进行精简，把回溯隐藏起来：

class Solution {
    public List<String> binaryTreePaths(TreeNode root) {
        List<String> result = new ArrayList();
        if(root == null) return result;
        String paths = "";
        dfs(root, paths, result);
        return result;
    }
    private void dfs(TreeNode root, String paths, List<String> result) {
        paths += root.val;
        if(root.left == null && root.right == null) {
            result.add(paths);
            return;
        }
        if(root.left != null) dfs(root.left, paths + "->", result);
        if(root.right != null) dfs(root.right, paths + "->", result);
    }
}