Solution.ts

/**
 * Definition for a binary tree node.
 * class TreeNode {
 *     val: number
 *     left: TreeNode | null
 *     right: TreeNode | null
 *     constructor(val?: number, left?: TreeNode | null, right?: TreeNode | null) {
 *         this.val = (val===undefined ? 0 : val)
 *         this.left = (left===undefined ? null : left)
 *         this.right = (right===undefined ? null : right)
 *     }
 * }
 */

function reverseOddLevels(root: TreeNode | null): TreeNode | null {
    const q: TreeNode[] = [root];
    for (let i = 0; q.length > 0; ++i) {
        if (i % 2) {
            for (let l = 0, r = q.length - 1; l < r; ++l, --r) {
                [q[l].val, q[r].val] = [q[r].val, q[l].val];
            }
        }
        const nq: TreeNode[] = [];
        for (const { left, right } of q) {
            if (left) {
                nq.push(left);
                nq.push(right);
            }
        }
        q.splice(0, q.length, ...nq);
    }
    return root;
}