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;    }};