103. Binary Tree Zigzag Level Order Traversal

Given a binary tree, return the zigzag level order traversal of its nodes' values. (ie, from left to right, then right to left for the next level and alternate between).
For example:

    3
   / \
  9  20
    /  \
   15   7
[
  [3],
  [20,9],
  [15,7]
]

这就是广度优先遍历的变种，使用队列，每个一层换一个顺序入队出队，哈哈js可以随便换队列方向

var zigzagLevelOrder = function(root) {
    if (!root) 
        return [];
    var q = [root];
    var res = [];
    var flag = true;
    while (q.length!==0) {
        var lastLength = q.length;
        res.push([]);
        if (flag) {
            for (let i = 0;i < lastLength;i++) {
                let temp = q.shift();
                res[res.length - 1].push(temp.val);
                if (temp.left)
                    q.push(temp.left);
                if (temp.right)
                    q.push(temp.right);
            }
        } else {
            for (let i = 0;i < lastLength;i++) {
                let temp = q.pop();
                res[res.length - 1].push(temp.val);
                if (temp.right)
                    q.unshift(temp.right);
                if (temp.left)
                    q.unshift(temp.left);
            }
        }
        flag = !flag;
    }
    return res;
};