9/6刷题二叉树的层次遍历II

1.二叉树的层次遍历II

思路一：利用队列的层次遍历+reverse

/**
 * 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) {
        vector<vector<int>> res;
        if(root==nullptr) return res;
        queue<TreeNode*> s1;
        vector<int> cur;
        int num=1;
        int curNum=0;
        s1.push(root);
        while(!s1.empty())
        {
            for(int i=0;i<num;i++)
            {
                TreeNode* p=s1.front();
                cur.push_back(p->val);
                s1.pop();
                if(p->left) 
                {
                    s1.push(p->left);
                    curNum++;
                }
                if(p->right)
                {
                    s1.push(p->right);
                    curNum++;
                }
            }
            res.push_back(cur);
            cur.clear();
            num=curNum;
            curNum=0;
        }
        reverse(res.begin(),res.end());
        return res;
    }
};