107. Binary Tree Level Order Traversal II

最新推荐文章于 2021-11-08 16:28:18 发布

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最新推荐文章于 2021-11-08 16:28:18 发布

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分类专栏： leetcode代码笔记

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本文介绍了一种算法，用于实现二叉树的从叶节点到根节点的层次遍历，并提供了两种实现方法：一种使用队列进行迭代，另一种采用递归方式。这两种方法最终都将结果反转以达到从下向上的层次遍历效果。

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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,null,null,15,7],

return its bottom-up level order traversal as:

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

class Solution {
public:
    vector<vector<int>> levelOrderBottom(TreeNode* root) {
        queue<TreeNode*> q;
        vector<vector<int>> res;         //！！！
        if(root == NULL)
        return res;     //不能写return 0! could not convert '0l' from 'long int' to 'std::vector<std::vector<int> >
        q.push(root);
        while(!q.empty())
        {
            vector<int> v;
            int x = q.size();        //下面的size会变！
            for(int i=0; i<x; i++)
            {
                TreeNode* p = q.front();
                v.push_back(p->val);
                q.pop();
                if(p->left)
                q.push(p->left);
                if(p->right)
                q.push(p->right);
                
            }
            res.push_back(v);          //// when the counter equals zero, add current v to result
            
        }

        vector<vector<int>> result;       //或者也可以用     reverse(res.begin(),res.end());;
        while(res.size()>0)
        {
            result.push_back(res.back());        //!!!!!
            res.pop_back();
        }
        return result;
        
        
    }
};

递归方法，更难理解。。

vector<vector<int> > levelOrder(TreeNode *root) {
	vector<vector<int> > retVal;

	levelOrder(root, retVal, 0);

	reverse(retVal.begin(), retVal.end());

	return retVal;
}

void levelOrder(TreeNode* root, vector<vector<int> > &v, int currLevel) {
	if (root == NULL) {
		return;
	}

	if (v.empty() || currLevel > (v.size() - 1)) {
		v.push_back(vector<int>());
	}

	v[currLevel].push_back(root->val);

	levelOrder(root->left, v, currLevel + 1);
	levelOrder(root->right, v, currLevel + 1);
}