113. Path Sum II

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]
]

public IList<IList<int>> PathSum(TreeNode root, int sum) {
        IList<IList<int>> res = new List<IList<int>>();
        BackTracking(root,sum,new List<int>(),res);
        return res;
    }
    
    public void BackTracking(TreeNode root, int sum, IList<int> cur,IList<IList<int>> res )
    {
        if(root != null)
        {
            if(root.left == null && root.right == null)//leaf check
            {
                if(sum == root.val)
                {
                    cur.Add(root.val);
                    res.Add(new List<int>(cur));
                }
            }
            else
            {
                cur.Add(root.val);
                BackTracking(root.left,sum - root.val,new List<int>(cur),res);
                BackTracking(root.right,sum - root.val,new List<int>(cur),res);
            }
        }
    }