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Find Bottom Left Tree Value⚓︎

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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, 10^4].
  • -2^31 <= Node.val <= 2^31 - 1

Solution⚓︎

Recursive Solution⚓︎

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/**
 * 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 {
private:
    int res = 0;
    int maxDepth = 0;
    void traversal(TreeNode* root, int depth) {
        if (!root) return;
        if (depth > maxDepth) {
            maxDepth = depth;
            res = root->val;
        }
        traversal(root->left, depth + 1);
        traversal(root->right, depth + 1);
    }
public:
    int findBottomLeftValue(TreeNode* root) {
        traversal(root, 1);
        return res;
    }
};

Iterative Solution (Level-order)⚓︎

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/**
 * 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) {
        int res;
        queue<TreeNode*> q;
        if (root) q.push(root);

        while (!q.empty()) {
            int len = q.size();
            for (int i = 0; i < len; i++) {
                auto t = q.front();
                q.pop();
                if (i == 0) res = t->val;
                if (t->left) q.push(t->left);
                if (t->right) q.push(t->right);
            }
        }
        return res;
    }
};