链接:https://leetcode.com/problems/sum-of-left-leaves/
原题:
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.
分析：这种题就是递归，，

public class Solution {
    public int sumOfLeftLeaves(TreeNode root) {
        if (root == null) {
            return 0;
        }
        int num =0;
        if(root.left!=null && root.left.left == null && root.left.right == null){
            num+=root.left.val;
        }
        return num + sumOfLeftLeaves(root.left) + sumOfLeftLeaves(root.right);
    }
}