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,#,#,15,7},

    3
   / 
  9  20
    /  
   15   7

return its bottom-up level order traversal as:

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

先查看有几层，然后填进去。

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<vector<int> > re;
    int levv;
    vector<vector<int> > levelOrderBottom(TreeNode *root) {
        if(root == NULL)return re;
        levv = level(root);

        for(int i = 0 ; i < levv ;i++)
        {
            vector<int> vec;
            re.push_back(vec);
        }
        bottom(root,1);
        
        return re;
    }
    void bottom(TreeNode * root ,int lev)
    {
        if(root == NULL)return;
        re[levv - lev].push_back(root->val);
        bottom(root->left,lev+1);
        bottom(root->right,lev+1);
    }
    int level(TreeNode *root)
    {
        if(root->left == NULL && root ->right == NULL)return 1;
        else if(root->left == NULL) return level(root->right)+1;
        else if(root->right == NULL) return level(root->left)+1;
        else
        return max(level(root->left),level(root->right))+1;
    }
};