C++-Binary Tree Level Order Trav

Given a binary tree, return the bottom-up level order traversal of its nodes' values. (ie, from left to right, level by level from leaf to root).

For example:
Given binary tree [3,9,20,null,null,15,7],

    3
   / \
  9  20
    /  \
   15   7

return its bottom-up level order traversal as:

[
  [15,7],
  [9,20],
  [3]
]

二叉树的层次遍历，不过需要保持每一层的数据，可以通过vector配合队列实现，实现:

class Solution {
public:
    vector<vector<int>> levelOrderBottom(TreeNode* root) {
        vector<vector<int>> vvi;
        if (root == NULL) {
            return vvi;
        }
        queue<TreeNode *> queue;
        queue.push(root);
        while (!queue.empty()) {
            vector<int> vi;
            int size = (int)queue.size();
            for (int i = 0; i < size; i++) {
                TreeNode *node = queue.front();
                queue.pop();
                if (node->left != NULL) {
                    queue.push(node->left);
                }
                if (node->right != NULL) {
                    queue.push(node->right);
                }
                vi.push_back(node->val);
            }
            vvi.push_back(vi);
        }
        reverse(vvi.begin(), vvi.end());
        return vvi;
    }
};