【剑指Offer】对称的二叉树

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题目

请实现一个函数，用来判断一棵二叉树是不是对称的。如果一棵二叉树和它的镜像一样，那么它是对称的。

实现

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public class TreeNode {
    int val = 0;
    TreeNode left = null;
    TreeNode right = null;
    
    public TreeNode(int val) {
        this.val = val;
    }
}
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public boolean isSymmetrical(TreeNode pRoot) {
    if (pRoot == null)
        return true;

    return isSymmetricalHelper(pRoot.left, pRoot.right);
}

private boolean isSymmetricalHelper(TreeNode left, TreeNode right) {
    if (left == null && right == null)
        return true;
    else if (left == null || right == null)
        return false;
    else if (left.val != right.val)
        return false;

    return isSymmetricalHelper(left.left, right.right) && isSymmetricalHelper(left.right, right.left);
}