layer()计算当前树的最大高度，然后从第一层依次递归（先左树，再右树）寻找到最大高度的层次，这个层次的第一个从左到右第一个节点就是bottomleft 。
/**
* Definition for a binary tree node.
* struct TreeNode {
* int val;
* struct TreeNode *left;
* struct TreeNode *right;
* };
*/
int layer(struct TreeNode* node) {
if (node == NULL) return 0;
int l = layer(node->left);
int r = layer(node->right);
return (l > r ? l + 1 : r + 1);
}
bool traverse(struct TreeNode* node, int layer, int target, int* res) {
if (node == NULL) return false;
if (target == layer) {
*res = node->val;
return true;
}
if(true == traverse(node->left, layer + 1, target, res) ||
true == traverse(node->right, layer + 1, target, res) ) {
return true;
}
return false;
}
int findBottomLeftValue(struct TreeNode* root) {
int l = layer(root);
int res = 0;
traverse(root, 1, l, &res);
return res;
}