LintCode 二叉树的层次遍历 II

中等 二叉树的层次遍历
II
查看执行结果

42%

通过

给出一棵二叉树，返回其节点值从底向上的层次序遍历（按从叶节点所在层到根节点所在的层遍历，然后逐层从左往右遍历）

您在真实的面试中是否遇到过这个题？

Yes

例子

给出一棵二叉树 {3,9,20,#,#,15,7},

    3
   / \
  9  20
    /  \
   15   7

依照从下往上的层次遍历为：

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

]

asd

/**
 * Definition of TreeNode:
 * class TreeNode {
 * public:
 *     int val;
 *     TreeNode *left, *right;
 *     TreeNode(int val) {
 *         this->val = val;
 *         this->left = this->right = NULL;
 *     }
 * }
 */

class Solution {
    /**
     * @param root : The root of binary tree.
     * @return : buttom-up level order a list of lists of integer
     */
public:
    vector<vector<int>> levelOrderBottom(TreeNode *root) {
         vector<vector<int>> res;
        if(root == nullptr) {
            return res;
        }
        vector<int> temp;
        queue<TreeNode*> q;
        stack<vector<int>> s;
        q.push(root);
        int i = 1;// points every level
        int j = 0;// lost point every level
        while(!q.empty()) {
            TreeNode *p = q.front();
            q.pop();
            if (p==nullptr) {
                ++j;
            }
            else {
               temp.push_back(p->val);
                q.push(p->left);
                q.push(p->right);
            }
            if (i == (temp.size() + j) && temp.size()!=0) {
                s.push(temp);
                temp.clear();
                i*=2;
                j*=2;
            }
        }
        while(!s.empty()) {
            res.push_back(s.top());
            s.pop();
        }
        return res;
    }
};