2016年2月26日 星期五

LEET code -- Binary Tree Level Order Traversal II

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},

    3
   / \
  9  20
    /  \
   15   7

return its bottom-up level order traversal as:

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


confused what "{1,#,2,3}" means? > read more on how binary tree is serialized on OJ.


queue 的STL 注意是push 跟front 還有pop

reverse也是STL


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/**
 * 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>> ans;//initial ,empty
        queue<TreeNode*> parse;
        if(root==NULL)return ans;
        parse.push(root);
        while(parse.size()>0){
            vector<int> level;//local level
            for(int i=parse.size();i>0;i--){
                TreeNode * front = parse.front();
                level.push_back(front->val);
                if(front->left!=NULL)parse.push(front->left);
                if(front->right!=NULL)parse.push(front->right);
                parse.pop();
            }
            ans.push_back(level);
            
        }
        reverse(ans.begin(),ans.end());
        
    }
};

---------------------------
recursive



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class Solution {
public:

    vector< vector<int> > result;

    void dfs(TreeNode *root, int level)
    {
        if(!root) return;

        if(level == result.size()) 
            result.push_back( vector<int>() );

        vector<int> &vec = result[level];
        vec.push_back(root->val);

        dfs(root->left, level+1);
        dfs(root->right, level+1);
    }

    vector<vector<int>> levelOrderBottom(TreeNode* root)
    {
        if(!root) return vector< vector<int> >();

        dfs(root, 0);

        return vector< vector<int> >(result.rbegin(), result.rend());
    }
};

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