107. Binary Tree Level Order Traversal II

最新推荐文章于 2021-08-10 13:02:56 发布

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本文介绍了一种算法，用于实现二叉树的层次遍历并返回从叶节点到根节点的层次顺序结果。该算法使用队列进行层级区分，并在遍历完成后反转结果以达到题目要求。

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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},

return its bottom-up level order traversal as:

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

和I差不多，只是需要把最后遍历结果数组翻转一下

/**
 * 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> > levelOrderBottom(TreeNode *root) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        vector<vector<int> >res;
        if(root == NULL)return res;
        queue<TreeNode*> myqueue;
        myqueue.push(root);
        myqueue.push(NULL);//NULL是层与层之间间隔标志
        vector<int> level;
        while(myqueue.empty() == false)
        {
            TreeNode *p = myqueue.front();
            myqueue.pop();
            if(p != NULL)
            {
                level.push_back(p->val);
                if(p->left)myqueue.push(p->left);
                if(p->right)myqueue.push(p->right);
            }
            else
            {
                res.push_back(level);
                if(myqueue.empty() == false)
                {
                    level.clear();
                    myqueue.push(NULL);
                }
            }
        }
        reverse(res.begin(), res.end());
        return res;
    }
};