- 寻找左子树的值的和；
- 遍历
int sumOfLeftLeaves(TreeNode* root) {
if(! root)return 0;
int sum =0;
sum += sumofleft(root->left,true);
sum +=sumofleft(root->right,false);
return sum;
}
int sumofleft(TreeNode* root, bool left){
if(!root) return 0;
if(! root->left && !root->right && left)
return root->val;
return sumofleft(root->left,true) + sumofleft(root->right,false);
}
