二叉树和为某一值的路径

 

 （1） DFS

    List<List<Integer>> res = new LinkedList<List<Integer>>();
    Deque<Integer> path = new LinkedList<>();
    public List<List<Integer>> pathSum(TreeNode root, int target) {
        dfs(root, target);
        return res;
    }
    public void dfs(TreeNode root, int target){
        if(root == null) return;
        path.addLast(root.val);
        target -= root.val;
        if(root.left == null && root.right == null && target ==0){
            res.add(new LinkedList<Integer>(path) );
        }
        dfs(root.left, target);
        dfs(root.right, target);
        path.pollLast();
    }

 （2）BFS +哈希表记录每个节点的父节点。

List<List<Integer>> res = new LinkedList<List<Integer>>();
    Map<TreeNode, TreeNode> map = new HashMap<TreeNode,TreeNode>();
    public List<List<Integer>> pathSum(TreeNode root, int target) {
        if(root == null ) return res;
        Queue<TreeNode> nodeQueue = new LinkedList<>();
        Queue<Integer> sumQueue = new LinkedList<>();
        nodeQueue.offer(root);
        sumQueue.offer(0);
        while(!nodeQueue.isEmpty()){
            TreeNode node = nodeQueue.poll();
            int tmp = sumQueue.poll() + node.val;
            if(node.left == null && node.right == null && tmp == target){
                getPath(node);
            }
            if( node.left != null){
                map.put(node.left, node);
                nodeQueue.offer(node.left);
                sumQueue.offer(tmp);
            }
            if( node.right != null){
                map.put(node.right, node);
                nodeQueue.offer(node.right);
                sumQueue.offer(tmp);
            }
        }
       return res;
    }

    private void getPath(TreeNode node) {
        LinkedList<Integer> list = new LinkedList<>();
        while(node != null){
            list.add(0, node.val);
            node = map.get(node);
        }
        res.add(list);
    }