BFS Binary Tree Level Order Traversal

思路：

BFS。

两个队列，一个保存当前层的所有节点，一个保存下一层的所有节点。

std::swap()函数：

C++98: <algorithm>, C++11: <utility>

在C++ 98中的定义：

// defined in <algorithm> before C++11

template <class T> void swap (T& a, T& b);

在C++ 11中的定义：

non-array (1)	template <class T> void swap (T& a, T& b) noexcept (is_nothrow_move_constructible<T>::value && is_nothrow_move_assignable<T>::value);
array (2)	template <class T, size_t N> void swap(T (&a)[N], T (&b)[N]) noexcept (noexcept(swap(*a,*b)));

std::queue：FIFO队列容器

Member functions

(constructor)

Construct queue  (public member function )

empty

Test whether container is empty  (public member function )

size

Return size  (public member function )

front

Access next element  (public member function )

back

Access last element  (public member function )

push

Insert element  (public member function )

emplace 

Construct and insert element  (public member function )

pop

Remove next element  (public member function )

swap 

Swap contents  (public member function )

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<vector<int>> levelOrder(TreeNode* root) {
        vector<vector<int>> res;
        if(root == NULL) return res;
        queue<TreeNode*> current, nextLevel;
        current.push(root);
        vector<int> level;
        while(!current.empty()) {
            while(!current.empty()) {
                TreeNode *node = current.front();
                current.pop();
                level.push_back(node->val);
                if(node->left != NULL) nextLevel.push(node->left);
                if(node->right != NULL) nextLevel.push(node->right);
            }
            res.push_back(level);
            level.clear();
            swap(current, nextLevel);
        }
        return res;
    }
};