简单题。现在对于这种递归已经很熟练了啊。

/** * Definition for binary tree * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */public class Solution {    public ArrayList
     
      
       > pathSum(TreeNode root, int sum) {        // Start typing your Java solution below        // DO NOT write main() function        ArrayList
       
        
         > ret = new ArrayList
         
          
           >(); ArrayList
           
             arr = new ArrayList
            
             (); PathSum(root, sum, arr, ret); return ret; } public void PathSum(TreeNode root, int sum, ArrayList
             
               arr, ArrayList
              
               
                > ans) { if (root == null) return; if (root.left == null && root.right == null && root.val == sum) { arr.add(root.val); ArrayList
                
                  tmp = new ArrayList
                 
                  (); for (int i = 0; i < arr.size(); i++) { tmp.add(arr.get(i)); } ans.add(tmp); arr.remove(arr.size() - 1); } else { arr.add(root.val); PathSum(root.left, sum - root.val, arr, ans); arr.remove(arr.size() - 1); arr.add(root.val); PathSum(root.right, sum - root.val, arr, ans); arr.remove(arr.size() - 1); } }}