SymmetricTree.java

package easy;

/**
 * Given a binary tree, check whether it is a mirror of itself (ie, symmetric around its center).
 * For example, this binary tree [1,2,2,3,4,4,3] is symmetric:
 *     1
 *    / \
 *   2   2
 *  / \ / \
 * 3  4 4  3
 * But the following [1,2,2,null,3,null,3] is not:
 *     1
 *    / \
 *   2   2
 *    \   \
 *    3    3
 * Note:
 * Bonus points if you could solve it both recursively and iteratively.
 *
 * 解决思路:
 * 是否对称
 * 递归调用比较left.left和right.right、left.right和right.left
 * 判断是否都为空返回true, 其中一方为空返回false
 * 比较left.val和right.val
 *
 * Created by huajun on 17-7-21.
 */
public class SymmetricTree {

    public boolean isSymmetric(TreeNode root) {
        return isMirror(root, root);
    }

    public boolean isMirror(TreeNode left, TreeNode right){
        if (left == null && right == null) return true;
        if (left == null || right == null) return false;
        return (left.val == right.val)
                && isMirror(left.left, right.right)
                && isMirror(left.right, right.left);
    }

    private class TreeNode{
        int val;
        TreeNode left;
        TreeNode right;
        TreeNode(int x) { val = x; }
    }
}