[LeetCode] Find Largest Value in Each Tree Row

 

You need to find the largest value in each row of a binary tree.

Example:

Input: 

          1
         / \
        3   2
       / \   \  
      5   3   9 

Output: [1, 3, 9]

找出二叉树中每一层最大的元素。

思路：利用层次遍历找出每一层最大元素即可。

/**
 * 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<int> largestValues(TreeNode* root) {
        vector<int> res;
        if (root == nullptr)
            return res;
        queue<TreeNode*> q;
        q.push(root);
        while (!q.empty()) {
            int n = q.size();
            int maxVal = INT_MIN;
            for (int i = 0; i < n; i++) {
                TreeNode* node = q.front();
                q.pop();
                maxVal = max(maxVal, node->val);
                if (node->left != nullptr)
                    q.push(node->left);
                if (node->right != nullptr)
                    q.push(node->right);
            }
            res.push_back(maxVal);
        }
        return res;
    }
};
// 14 ms

 思路：使用递归dfs来查找每层最大值。

/**
 * 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<int> largestValues(TreeNode* root) {
        vector<int> res;
        dfs(root, 0, res);
        return res;
    }
    
    void dfs(TreeNode* node, int height, vector<int>& res) {
        if (node == nullptr)
            return;
        if (height >= res.size()) {
            res.push_back(node->val);
        }
        else {
            res[height] = max(res[height], node->val);
        }
        dfs(node->left, height + 1, res);
        dfs(node->right, height + 1, res);
    }
};
// 13 ms