从根节点开始按层次遍历一棵二叉树

思路：广搜即可

class Solution {
public:
    vector<vector<int> > levelOrder(TreeNode *root) {
        vector<vector<int> > resVec;
        if (!root)
            return resVec;
        queue<TreeNode*> nodeQue;
        nodeQue.push(root);
        while (!nodeQue.empty())
        {
            vector<int> subVec;
            int qSize = nodeQue.size();
            for (size_t i = 0; i < qSize; i++)
            {
                if (nodeQue.front()->left)
                    nodeQue.push(nodeQue.front()->left);
                if (nodeQue.front()->right)
                    nodeQue.push(nodeQue.front()->right);
                subVec.push_back(nodeQue.front()->val);
                nodeQue.pop();
            }
            resVec.push_back(subVec);
        }
        return resVec;
    }
};

Binary Tree Level Order Traversal

原文：http://www.cnblogs.com/flyjameschen/p/fab978d81dfb209c08ab3a42af9d0823.html