力扣513. 找树左下角的值（层序+递归）

层序遍历法

/**
 * 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*> que;
        if(root) que.push(root);
        int result;
        while(!que.empty()) {
            result = que.front()->val;
            int size = que.size();
            while(size--) {
                TreeNode* node = que.front();
                que.pop();
                if(node->left) que.push(node->left);
                if(node->right) que.push(node->right);
            }
        }
        return result;
    }
};

递归遍历

/**
 * 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:
    void traversal(TreeNode* node, int depth, int& result) {
        if(!node->left && !node->right) {
            if(depth > max_depth) {
                max_depth = depth;
                result = node->val;
            }
            return;
        }
        if(node->left) {
            depth++;
            traversal(node->left, depth, result);
            depth--;
        }
        if(node->right) {
            depth++;
            traversal(node->right, depth, result);
            depth--;
        }
    }

    int findBottomLeftValue(TreeNode* root) {
        int result;
        traversal(root, 0, result);
        return result;
    }
private:
    int max_depth = INT_MIN;
};