LN : leetcode 515 Find Largest Value in Each Tree Row

lc 515 Find Largest Value in Each Tree Row

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

Example:
Input:

Output: [1, 3, 9]

BFS Accepted

相比之前的几道用到BFS算法的题目，这题相对来说会繁琐一些，需要用变量记录每一个节点所在的层数l，当前比较大小的层数m，重要的是厘清思路，其实也挺简单的，就是容易脑子搭线，陷入僵局。

/**
 * 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) {
        if (!root)  return {};
        queue<TreeNode*> q;
        queue<int> level;
        vector<int> ans;
        q.push(root);
        level.push(0);
        int m = -1;
        while(!q.empty()) {
            TreeNode* temp = q.front();
            q.pop();
            int l = level.front();
            level.pop();
            if (temp->left) {
                q.push(temp->left);
                level.push(l+1);
            }
            if (temp->right) {
                q.push(temp->right);
                level.push(l+1);
            }
            if (l > m) {
                m = l;
                ans.push_back(temp->val);
            } else {
                ans[l] = max(ans[l], temp->val);
            }
        }
        return ans;
    }
};