leetcode-513

BFS

開始做BFS的題目， 題目鏈接： https://leetcode-cn.com/problems/find-bottom-left-tree-value/

1. 想到的是標記層數，記錄最新一層的的第一個，最後一次記錄即爲需要的。

/**
 * 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:
    int findBottomLeftValue(TreeNode* root) {
        queue<TreeNode*> work;
        work.push(root);
        map<TreeNode*, int> dis;
        dis[root] = 0;

        int cur = 0;
        int ans = root->val;
        while (!work.empty()) {
            TreeNode* tmp = work.front();
            work.pop();
            if (dis[tmp] != cur) {
                ans = tmp->val;
                cur = dis[tmp];
            }

            if (tmp->left != nullptr) {
                work.push(tmp->left);
                dis[tmp->left] = dis[tmp] + 1;
            }

            if (tmp->right != nullptr) {
                work.push(tmp->right);
                dis[tmp->right] = dis[tmp] + 1;
            }
        }

        return ans;
    }
};

2. 參考評論代碼，從右往左遍歷，最後一個即爲需要的。

/**
 * 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:
    int findBottomLeftValue(TreeNode* root) {
        queue<TreeNode*> work;
        work.push(root);

        TreeNode* ans = root;
        while (!work.empty()) {
            ans = work.front();
            work.pop();
            if (ans->right != nullptr) {
                work.push(ans->right);
            }

            if (ans->left != nullptr) {
                work.push(ans->left);
            }
        }

        return ans->val;
    }
};