[leetcode-404-Sum of Left Leaves]

Find the sum of all left leaves in a given binary tree.

Example:

    3
   / \
  9  20
    /  \
   15   7

There are two left leaves in the binary tree, with values 9 and 15 respectively. Return 24.
思路：
递归遍历，判断是左叶子加入sum.

 void tr(TreeNode* root, int& sum)
     {
         if (root == NULL)return;
         if (root->left != NULL && root->left->left == NULL && root->left->right == NULL)sum += root->left->val;
         tr(root->left, sum);
         tr(root->right, sum);
     }
     int sumOfLeftLeaves(TreeNode* root)
     {
         if (root == NULL)return 0;
         int sum = 0;
         tr(root, sum);
         return sum;
     }