404. 左叶子之和

思路：

https://www.bilibili.com/video/BV1GY4y1K7z8/?vd_source=cc3333a27046bad449a2b6818cc4149c
感觉挺难想的，都是根据上层推导过来的

struct TreeNode {
	int val;
	TreeNode* left;
	TreeNode* right;
	TreeNode(int x):val(x),left(NULL),right(NULL){}
};

class Solution {
public:
	int TreeNodeSum(TreeNode* root) {
		if (root == NULL)return 0;
		if (root->left == NULL && root->right == NULL)return 0;
		int leftsum = TreeNodeSum(root->left);
		if (root->left != NULL && root->left->left == NULL && root->left->right == NULL) {
			leftsum = root->left->val;
		}
		int rightsum = TreeNodeSum(root->right);
		return leftsum + rightsum;
	}
};

int main() {
	TreeNode* A = new TreeNode(3);
	TreeNode* B = new TreeNode(9);
	TreeNode* C = new TreeNode(20);
	TreeNode* D = new TreeNode(15);
	TreeNode* E = new TreeNode(17);
	A->left = B;
	A->right = C;
	C->left = D;
	C->right = E;
	Solution ss;
	cout << ss.TreeNodeSum(A) << endl;
	return 0;
}