[LeetCode] 404. Sum of Left Leaves

原题链接：https://leetcode.com/problems/sum-of-left-leaves/

1. 题目介绍

Find the sum of all left leaves in a given binary tree.
给出一个二叉树，计算所有左叶子节点的val值之和。
Example:

There are two left leaves in the binary tree, with values 9 and 15 respectively. Return 24.
有两个左叶子节点，分别是 9 和15。返回他们的总和24.

2. 解题思路

这道题目可以使用深度优先搜索 + 递归做。按照深度优先搜索的顺序遍历每个节点，如果发现这个节点的左节点是叶子节点的话，就加上该叶子节点的val值。

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public int sumOfLeftLeaves(TreeNode root) {
        if(root == null ){
            return 0;
        }
        
        int k = 0;
        //判断root的左节点是否为叶子节点
        if(root.left != null && root.left.left == null && root.left.right == null){
            k = root.left.val;
        }
      
        return sumOfLeftLeaves(root.left) + sumOfLeftLeaves(root.right) + k; 
    }
}