Find the sum of all left leaves in a given binary tree.

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Example:
    3
   / \
  9  20
    /  \
   15   7
There are two left leaves in the binary tree, with values 9 and 15 respectively. Return 24.

碰到树的问题，一般用递归做起来最方便。
这道题主要是如何判断一个节点是不是left leaf, 所以我声明了一个helper函数，把parent传进去了。
然后给根节点弄了个dummy的parent。因为根节点不是left leaf。所有dummy.right = root.

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public int sumOfLeftLeaves(TreeNode root) {
    TreeNode dummy = new TreeNode(0);
    dummy.right = root;
    return helper(root, dummy);
}
public int helper(TreeNode node, TreeNode parent) {
    if (node == null)
        return 0;
    if (node.left == null && node.right == null && node == parent.left)
        return node.val;
    int left = helper(node.left, node);
    int right = helper(node.right, node);
    return left + right;
}