[leetcode] Sum Root to Leaf Numbers

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public class Solution { public int sumNumbers(TreeNode root) { return sumNumbers_rec(root, ""); } private int sumNumbers_rec(TreeNode root, String upper){ if (root == null) return 0; if (root.left == null && root.right == null) return (new Integer(upper+root.val)).intValue(); return sumNumbers_rec(root.left, upper+root.val)+sumNumbers_rec(root.right, upper+root.val); } }