404. Sum of Left Leaves

题目描述：

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.

解题思路：

这个题不断遍历整棵树找到左子树就可以了。时间复杂度O（n）

答案详解：

 /**

 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    int total = 0;
    void preorder(TreeNode* root)
    {
        if(root)
        {
            if( root->left==NULL && root->right==NULL )
                total = total + root->val;
            
            preorder(root->left);
            if(root->right!=NULL)
                if(root->right->left!=NULL || root->right->right !=NULL)
                    preorder(root->right);
        }
    }
    int sumOfLeftLeaves(TreeNode* root) {
        if(root&&root->right==NULL&&root->left==NULL)
            return 0;
        preorder(root);
        return total;
    }
};