LeetCode: 107_Binary Tree Level Order Traversal II | 二叉树自底向上的层次遍历 | Easy

本题和上题一样同属于层次遍历，不同的是本题从底层往上遍历，如下：

代码如下：

struct TreeNode {
    int            val;
    TreeNode*    left;
    TreeNode*    right;
    TreeNode():val(0),left(NULL),right(NULL){}
    TreeNode(int x): val(x), left(NULL),right(NULL) {}
};

vector<vector<int> > levelOrderTraversalII(TreeNode *root) //非递归的中序遍历(用栈实现)
{

    queue<TreeNode *> tree_queue;
    vector<vector<int> > tree_vector, tree_rvector;
    vector<int> svector;
    int nCount = 0;

    if (NULL == root) {
        return tree_vector;
    }
    TreeNode *pTemp = root;
    tree_queue.push(root);
    tree_queue.push(NULL); //the end of one level.

    while (true) {
        TreeNode *pTemp = tree_queue.front();
        tree_queue.pop();

        if (!pTemp) { //get the null, put vector<> to vector<vector<>>
            tree_vector.push_back(svector);
            nCount ++;
            svector.clear();
            if (tree_queue.empty())
                break;
            tree_queue.push(NULL);
        }
        else {
            svector.push_back(pTemp->val);
            if (pTemp->left) 
                tree_queue.push(pTemp->left);
            if (pTemp->right)
                tree_queue.push(pTemp->right);
        }
    }
    while (nCount > 0){
        svector = tree_vector.at(nCount - 1);
        tree_rvector.push_back(svector);
        nCount --;
    }
    return tree_rvector;
}