(二叉树 BFS) leetcode 107. Binary Tree Level Order Traversal II

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]
]

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这个就是二叉树的层序遍历，和leetcode102. Binary Tree Level Order Traversal类似，只是要把最后得到的二维数组逆转一下就成。

C++代码：

/**
 * 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>> levelOrderBottom(TreeNode* root) {
        if(!root) return {};
        queue<TreeNode*> q;
        q.push(root);
        vector<vector<int> > vec;
        while(!q.empty()){
            vector<int> v;;
            for(int i = q.size(); i > 0; i--){
                auto t = q.front();
                q.pop();
                v.push_back(t->val);
                if(t->left) q.push(t->left);
                if(t->right) q.push(t->right);
            }
            vec.push_back(v);
        }
        reverse(vec.begin(),vec.end());
        return vec;
    }
};