116+connect_Queue.cpp

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

    Node() : val(0), left(NULL), right(NULL), next(NULL) {}

    Node(int _val) : val(_val), left(NULL), right(NULL), next(NULL) {}

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

class Solution {
public:
    Node* connect(Node* root) {
        queue<Node*> que;
        if (root != NULL) que.push(root);
        while (!que.empty()) {
            int size = que.size();
            Node* node;
            Node* prenode;

            for (int i = 0; i < size; ++i) {
                if (i == 0) {
                    prenode = que.front();
                    que.pop();
                    node = prenode;
                } else {
                    node = que.front();
                    que.pop();
                    prenode->next = node;
                    prenode = prenode->next;
                }
                if (node->left)     que.push(node->left);
                if (node->right)    que.push(node->right);
            }
            prenode->next = NULL;   //  node 或者 prenode 都可以
        }
        return root;
    }
};