404. Sum of Left Leaves

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

Example:

    3   / \  9  20    /  \   15   7There are two left leaves in the binary tree, with values 9 and 15 respectively. Return 24.

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