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;
}
};
