二叉树的层次遍历BFS

使用队列实现BFS
queue.peek()只获取队首元素不删除
queue.poll()获取并删除队首元素

/*
// Definition for a Node.
class Node {
    public int val;
    public Node left;
    public Node right;
    public Node next;

    public Node() {}
    
    public Node(int _val) {
        val = _val;
    }

    public Node(int _val, Node _left, Node _right, Node _next) {
        val = _val;
        left = _left;
        right = _right;
        next = _next;
    }
};
*/

class Solution {
    
    
    public Node connect(Node root) {
    
    
        //队列BFS
        if(root == null){
    
    
            return null;
        }
        Queue<Node> queue = new LinkedList<>();
        queue.add(root);
        while(!queue.isEmpty()){
    
    
            int l = queue.size();
            for(int i = 0;i < l;i++){
    
    
                Node node = queue.poll();
                if(i < l - 1){
    
    
                    node.next = queue.peek();
                }
                if(node.left != null){
    
    
                    queue.add(node.left);
                }
                if(node.right != null){
    
    
                    queue.add(node.right);
                }
            }
        }
        return root;
    }
}

/*
// Definition for a Node.
class Node {
    public int val;
    public Node left;
    public Node right;
    public Node next;

    public Node() {}
    
    public Node(int _val) {
        val = _val;
    }

    public Node(int _val, Node _left, Node _right, Node _next) {
        val = _val;
        left = _left;
        right = _right;
        next = _next;
    }
};
*/
class Solution(){
    
    
	public Node connect(Node root){
    
    
		if(root == null){
    
    
			return null;
		}
		Node leftmost = root;
		while(leftmost.left != null){
    
    
			Node head = leftmost;
			while(head != null){
    
    
				head.left.next = head.right;
				if(head.next != null){
    
    
					head.right.next = head.next.left;
				}
			head = head.next;
			}
			leftmost = leftmost.left;
		}
		return root;
	}
}