[每日 LeetCode] 404. Sum of Left Leaves

原文链接 [每日 LeetCode] 404. Sum of Left Leaves

Description:

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**.
​

思路：本题要求树中所有左叶子节点的值之和。采用递归方法，检查当前结点的左子结点是否是左结点，如果是的话，则返回左子叶的值加上对当前结点的右子结点调用递归的结果；如果不是的话，我们对左右子结点分别调用递归函数，返回二者之和。

C++ 代码

/**
 * 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 sumOfLeftLeaves(TreeNode* root) {
        if (!root)
            return 0;
        if (root->left && !root->left->left && !root->left->right) {
            return root->left->val + sumOfLeftLeaves(root->right);
        }
        return sumOfLeftLeaves(root->left) + sumOfLeftLeaves(root->right);
    }
};
​

运行时间：4ms

运行内存：13.8M