题目

思路

dfs

1. 值相等，且 left.left 对 right.right，left.right 对 right.left

bfs

成对入队，每次弹出两个节点比较

Java

class Solution {
    public boolean isSymmetric(TreeNode root) {
        if (root == null) {
            return true;
        }
        return isMirror(root.left, root.right);
    }

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

class Solution {
    public boolean isSymmetric(TreeNode root) {
        if (root == null) return true;

        Deque<TreeNode> q = new LinkedList<>();
        q.add(root.left);
        q.add(root.right);

        while (!q.isEmpty()) {
            TreeNode a = q.poll();
            TreeNode b = q.poll();

            if (a == null && b == null) continue;
            if (a == null || b == null) return false;
            if (a.val != b.val) return false;

            q.add(a.left);
            q.add(b.right);
            q.add(a.right);
            q.add(b.left);
        }
        return true;
    }
}