題目：

Given a binary tree and a sum, find all root-to-leaf paths where each path's sum equals the given sum. 

              5
             / \
            4   8
           /   / \
          11  13  4
         /  \    / \
        7    2  5   1

return 

[
   [5,4,11,2],
   [5,8,4,5]
]

思路:注意前序遍歷，返回前面一個狀態，遞迴

程式碼：

public class Solution {
    ArrayList<ArrayList<Integer>> res = new ArrayList<>();
    public ArrayList<ArrayList<Integer>> pathSum(TreeNode root, int sum) {
        ArrayList<Integer> list = new ArrayList<Integer>();
        paths(root, sum, list);
        return 
 
res;
    }
    private void paths(TreeNode root, int sum, ArrayList<Integer> list){
        if(root == null)
            return;
        if(root.left == null && root.right == null && sum - root.val == 0){
            list.add(root.val);
            res.add(new ArrayList<Integer>(list));
            list.remove(list.size() - 1
 
);
            return;
        }
        list.add(root.val);
        paths(root.left, sum - root.val, list);
        paths(root.right, sum - root.val, list);
        list.remove(list.size() - 1);
    }
}

Given a binary tree and a sum, find all root-to-leaf paths where each path's sum equals the given sum. 

              5
             / \
            4   8
           /   / \
          11  13  4
         /  \    / \
        7    2  5   1

return 

[
   [5,4,11,2],
   [5,8,4,5]
]