Find Bottom Left Tree Value

Description

Given the root of a binary tree, return the leftmost value in the last row of the tree.

Example 1:

Input: root = [2,1,3] Output: 1

Example 2:

Input: root = [1,2,3,4,null,5,6,null,null,7] Output: 7

Constraints:

• The number of nodes in the tree is in the range [1, 104].
• -231 <= Node.val <= 231 - 1

Code

Time Complexity: $O(n)$, Space Complexity: $O(n)$

From left to right： 使用 Binary Tree Level Order Traversal，若是由左到右，則用 firstLeft 去紀錄每一層的最左邊的 node。

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    int findBottomLeftValue(TreeNode* root) {
        queue<TreeNode*> q;
        q.push(root);
        TreeNode* firstLeft = nullptr;
        while(!q.empty()) {
            int n = q.size();
            for(int i = 0; i < n; i++) {
                auto child = q.front();
                if(i == 0) firstLeft = child;
                q.pop();
                if(child->left) q.push(child->left);
                if(child->right) q.push(child->right);
            }
        }
        return firstLeft->val;
    }
};

若是由右到左，就不需要額外的 pointer 了。

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    int findBottomLeftValue(TreeNode* root) {
        queue<TreeNode*> q;
        q.push(root);
        while(!q.empty()) {
            int n = q.size();
            for(int i = 0; i < n; i++) {
                root = q.front();
                q.pop();
                if(root->right) q.push(root->right);
                if(root->left) q.push(root->left);
            }
        }
        return root->val;
    }
};

Source

• Find Bottom Left Tree Value - LeetCode