LeetCode

二叉树的层序遍历

思路一：利用队列，将每一层节点放入队列，各层节点之间加入NULL隔开。

/**
 * 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) {
        vector<vector<int> > levelOrderVec;
        if (root == NULL)
            return levelOrderVec;
        queue<TreeNode *> treeQueue;
        treeQueue.push(root);
        treeQueue.push(NULL);
        vector<int> level;
        while (!treeQueue.empty())
        {
            TreeNode *tmp = treeQueue.front();
            treeQueue.pop();
            if (tmp != NULL)
            {
                level.push_back(tmp->val);
                if (tmp->left)
                    treeQueue.push(tmp->left);
                if (tmp->right)
                    treeQueue.push(tmp->right);
            }
            else
            {
                if (level.size() != 0)
                {
                    levelOrderVec.push_back(level);
                    treeQueue.push(NULL);
                }
                level.clear();
            }
        }
        
        reverse(levelOrderVec.begin(), levelOrderVec.end());
        
        return levelOrderVec;
    }
};

思路二：深度优先搜索

/**
 * 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) {
        DFS(root, 0);
        return vector<vector<int> > (res.rbegin(), res.rend());
    }
    
    void DFS(TreeNode *root, int level)
    {
        if (root == NULL) return;
        // 当前level不存在
        // 新建一个
        if (level == res.size())
        {
            res.push_back(vector<int>());
        }
        
        res[level].push_back(root->val);
        DFS(root->left, level+1);
        DFS(root->right, level+1);
    }
    
private:
    vector<vector<int> > res;
};