[LeetCode] Path Sum II

Problem:

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

For example:
Given the below binary tree and sum = 22,

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

return

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

Solution:O(n)

public class Solution {
   public ArrayList<ArrayList<Integer>> pathSum(TreeNode root, int sum) {
      ArrayList<ArrayList<Integer>> list = new ArrayList<>();
      if(root == null)
         return list;
            
      if(root.left == null && root.right == null){
         if(root.val == sum){
            ArrayList<Integer> s = new ArrayList<>();
            s.add(sum);
            list.add(s);
            return list;
         }

      }    
            
      if(root.left !=null){
         for(ArrayList<Integer> s:pathSum(root.left, sum-root.val)){

             s.add(0,root.val);
             list.add(s);
         }
      }    
        
      if(root.right !=null){
         for(ArrayList<Integer> s:pathSum(root.right, sum-root.val)){
            s.add(0,root.val);             

            list.add(s);
         }
      }    
      return list;
   }
}