对称二叉树(Symmetric Tree)

LeetCode传送门101. Symmetric Tree

题目

Given the root of a binary tree, check whether it is a mirror of itself (i.e., symmetric around its center).

给定一个二叉树，检查它是否是镜像对称的。

Example:

Input: root = [1,2,2,3,4,4,3]
Output: true

Input: root = [1,2,2,null,3,null,3]
Output: false

Constraints:

The number of nodes in the tree is in the range [1, 1000]. -100 <= Node.val <= 100

Follow up: Could you solve it both recursively and iteratively?

思考线

解题思路

可以递归遍历根节点的左右子树，检查左子树的左节点是否和右子树的右节点相等，右节点的左子树和左节点的右子树是否相等即可。

代码如下

function isSymmetric(root: TreeNode | null): boolean {
    if (!root) return true;
    return isS(root.left, root.right)

};
function isS(a, b) {
    if (!a && !b) return true;
    if (!a && b || !b && a) return false;
    if (a.val !== b.val) return false;
    return isS(a.left, b.right) && isS(a.right, b.left);
}

同样我们也可以用迭代的方式实现相同的功能

function isSymmetric(root: TreeNode | null): boolean {
    if (!root) return true;
    const stack = [root.left, root.right]
    while (stack.length) {
        const l = stack.shift();
        const r= stack.shift();
        if (!l && !r) continue;
        if (!l && r || !r && l) return false;
        if (l.val !== r.val) return false;
        stack.push(l.left)
        stack.push(r.right)
        stack.push(r.left)
        stack.push(l.right)
    }
    return true;

};

这就是我对本题的解法，如果有疑问或者更好的解答方式，欢迎留言互动。