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代码链接：

力扣（LeetCode）官网 - 全球极客挚爱的技术成长平台

思路：    

        万金油层次遍历，保存每一层的第一个元素返回就行了

我的代码：

/*** 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) {std::queue<TreeNode*> que;que.push(root);int res = 0;while(!que.empty()){int len = que.size();for(int i = 0; i < len; i++){TreeNode* node = que.front();que.pop();if(i == 0) res = node->val;if(node->left) que.push(node->left);if(node->right) que.push(node->right);}}return res;}
};

迭代回溯法：

class Solution {
public:int maxDepth = INT_MIN;int result;int findBottomLeftValue(TreeNode* root) {traversal(root, 1);return result;}void traversal(TreeNode* root, int depth){if(root->left == nullptr && root->right == nullptr){if(depth > maxDepth){maxDepth = depth;result = root->val;}return;}if(root->left){depth++;traversal(root->left, depth);depth--;}if(root->right){depth++;traversal(root->right, depth);depth--;}return;}
};