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.
    Runtime: 4 ms, faster than 75.60% of C++ online submissions for Sum of Left Leaves.

    1. /**
    2.  * Definition for a binary tree node.
    3.  * struct TreeNode {
    4.  *     int val;
    5.  *     TreeNode *left;
    6.  *     TreeNode *right;
    7.  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
    8.  * };
    9.  */
    10. class Solution {
    11. public:
    12.     int sumOfLeftLeaves(TreeNode* root) {
    13.         if (root == NULL) {
    14.             return 0;
    15.         }
    16.         int sum = 0;
    17.         if (root->left != NULL) {
    18.             if (root->left->left == NULL && root->left->right == NULL) { // isLeaf
    19.                 sum += root->left->val;
    20.             } else {
    21.                 sum += sumOfLeftLeaves(root->left);
    22.             }
    23.         }
    24.         if (root->right != NULL) {
    25.             sum += sumOfLeftLeaves(root->right);
    26.         }
    27.         return sum;
    28.     }
    29. };