Description

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]

Explain

這題題意一眼就知道要干嘛，言簡(jiǎn)意賅。就是要找樹的每一層的最大值。這里在leetcode的分類是DFS。然后我覺得這題用BFS來(lái)做更好做一點(diǎn)，也更好理解一點(diǎn)。我們只要遍歷每一層的節(jié)點(diǎn)，然后比較得出最大值，然后插入vector即可。下面上代碼

Code

/**
 * 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) return res;
        queue<TreeNode*> q;
        q.push(root);
        while(!q.empty()) {
            int len = q.size();
            int max = INT_MIN;
            for (int i = 0; i < len; i++) {
                TreeNode* cur = q.front();
                q.pop();
                if (cur->val > max) max = cur->val;
                if (cur->left) q.push(cur->left);
                if (cur->right) q.push(cur->right);
            }
            res.push_back(max);
        }
        return res;
    }
};