Binary Tree Level Order Traversal II【二叉树层序转换】

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本文介绍了一种从叶节点到根节点的层次遍历二叉树的方法，并提供了一个具体的C++实现示例。

PROBLEM:

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]
]

SOLVE:

/**
 * 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>> T;
        int level=0;
        levelOrder(root,T,level);
        std::reverse(T.begin(),T.end());
        return T;
    }
    void levelOrder(TreeNode* root,vector<vector<int>> &T,int curlevel){
        if(root==NULL) return;
        if(T.empty()||T.size()<curlevel+1)
            T.push_back(vector<int>());
        T[curlevel].push_back(root->val);
        levelOrder(root->left,T,curlevel+1);
        levelOrder(root->right,T,curlevel+1);
    }
};

注意：1、用引用传递容器；2、二维容器中压入一个空的一维容器：T.push_back(vector<int>())