####QUESTION: 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.
####EXPLAINATION:
该问题需要注意的地方其实就是根节点其实是不算的，需要注意区分左右节点的问题。
####SOLUTION:
public int sumOfLeftLeaves(TreeNode root) {
if(root==null) return 0;
if(root.left!=null&& root.left.left==null&& root.left.right==null){
return root.left.val+ sumOfLeftLeaves(root.right);
}
return sumOfLeftLeaves(root.left)+sumOfLeftLeaves(root.right);
}