513

513. Find Bottom Left Tree Value (Medium)

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

Example 1:

Input:
2

/ 

1   3
Output:
1

Example 2:

Input:
    1
   / \
  2   3
 /   / \
4   5   6
   /
  7

Output:
7

Note: You may assume the tree (i.e., the given root node) is not NULL.

Related Topics:
Tree, Depth-first Search, Breadth-first Search

Solution 1. Level-order Traversal

// OJ: https://leetcode.com/problems/find-bottom-left-tree-value/
// Author: github.com/lzl124631x
// Time: O(N)
// Space: O(N)
class Solution {
public:
    int findBottomLeftValue(TreeNode* root) {
        int ans = 0;
        queue<TreeNode*> q;
        q.push(root);
        while (q.size()) {
            int cnt = q.size();
            ans = q.front()->val;
            while (cnt--) {
                root = q.front();
                q.pop();
                if (root->left) q.push(root->left);
                if (root->right) q.push(root->right);
            }
        }
        return ans;
    }
};

Solution 2. Level-order Traversal (Right-to-left)

// OJ: https://leetcode.com/problems/find-bottom-left-tree-value/
// Author: github.com/lzl124631x
// Time: O(N)
// Space: O(N)
class Solution {
public:
    int findBottomLeftValue(TreeNode* root) {
        int ans = 0;
        queue<TreeNode*> q;
        q.push(root);
        while (q.size()) {
            root = q.front();
            ans = root->val;
            q.pop();
            if (root->right) q.push(root->right);
            if (root->left) q.push(root->left);
        }
        return ans;
    }
};

Solution 3. Pre-order Traversal

// OJ: https://leetcode.com/problems/find-bottom-left-tree-value/
// Author: github.com/lzl124631x
// Time: O(N)
// Space: O(H)
class Solution {
    int maxDepth = -1, ans = 0;
    void preorder(TreeNode *root, int depth) {
        if (!root) return;
        if (depth > maxDepth) {
            ans = root->val;
            maxDepth = depth;
        }
        preorder(root->left, depth + 1);
        preorder(root->right, depth + 1);
    }
public:
    int findBottomLeftValue(TreeNode* root) {
        preorder(root, 0);
        return ans;
    }
};