116. Populating-Next-Right-Pointers-in-Each-Node

/**
 * // Definition for a _Node.
 * function _Node(val, left, right, next) {
 *    this.val = val === undefined ? null : val;
 *    this.left = left === undefined ? null : left;
 *    this.right = right === undefined ? null : right;
 *    this.next = next === undefined ? null : next;
 * };
 */

/**
 * @param {_Node} root
 * @return {_Node}
 */
var connect = function(root) {
    
    if(!root) return root;
    let queue = [root]
    while(queue.length > 0){
        let n = queue.length;
        let prev = undefined;
        for(let i = 0; i < n; i++){
            let node = queue.shift();
            if(!prev)
                prev = node;
            else{
                prev.next = node;
                prev = node;
            }   
                
            if(node.left) queue.push(node.left)
            if(node.right) queue.push(node.right);
        }
    }
    
    return root;
};