Leetcode 107. Binary Tree Level Order Traversal II

文章作者：Tyan
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1. Description

Binary Tree Level Order Traversal II

2. Solution

• Version 1

/**
 * 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> levelOrderBottom(TreeNode* root) {
        vector> result;
        if(!root) {
            return result;
        }
        queue q;
        q.push(root);
        traverseLevel(q, result);
        reverse(result.begin(), result.end());
        return result;
    }
    
    
private:
    void traverseLevel(queue& q, vector>& result) {
        vector values;
        int size = q.size();
        while(size) {
            TreeNode* current = q.front();
            q.pop();
            if(current) {
                values.push_back(current->val);
                if(current->left) {
                    q.push(current->left);
                }
                if(current->right) {
                    q.push(current->right);
                }
            }
            size--;
        }
        result.push_back(values);
        if(!q.empty()) {
            traverseLevel(q, result);
        }
    }
};

• Version 2

/**
 * 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> levelOrderBottom(TreeNode* root) {
        vector> result;
        if(!root) {
            return result;
        }
        depthLevel(result, root, 0);
        reverse(result.begin(), result.end());
        return result;
    }
    
    
private:
    void depthLevel(vector>& result, TreeNode* root, int depth) {
        if(!root) {
            return;
        }
        depth++;
        if(depth > result.size()) {
            vector values;
            result.emplace_back(values);
        }
        depthLevel(result, root->left, depth);
        depthLevel(result, root->right, depth);
        result[depth - 1].push_back(root->val);
    }
};

Reference

1. https://leetcode.com/problems/binary-tree-level-order-traversal-ii/description/