814. Binary Tree Pruning 

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class Solution {
    public TreeNode pruneTree(TreeNode root) {
        if (root == null) {
            return null;
        }

        root.left = pruneTree(root.left);
        root.right = pruneTree(root.right);

        if (root.val == 0 && root.left == null && root.right == null) {
            // 仅值为 0 的叶子节点可消除
            return null;
        } else {
            return root;
        }
    }
}

References

814. Binary Tree Pruning
剑指 Offer II 047. 二叉树剪枝