[LeetCode]Binary Tree Level Order Traversal II

CiaoLiang

于 2015-12-29 16:33:37 发布

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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。该算法首先计算二叉树的深度，然后通过递归方式收集每个层级的节点值。

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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]
]

/**
 * 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) {
        int d = depth(root);
        vector<vector<int>> ret(d);
        levelOrder(ret, root, d-1);
        return ret;
    }
    int depth(TreeNode *root) {
        if (!root) return 0;
        return max(depth(root->left),depth(root->right))+1;
    }
    
    void levelOrder(vector<vector<int>> &ans, TreeNode *node, int level) {
        if (!node) return;
        ans[level].push_back(node->val);
        levelOrder(ans,node->left,level-1);
        levelOrder(ans,node->right,level-1);
    }
};