113. Path Sum II（求等于某个数的所有路径）

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]
]

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