leetcode Sum of Left Leaves

Description

Find the sum of all left leaves in a given binary tree.

Example:

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    3
   / \
  9  20
      /  \
   15   7

There are two left leaves in the binary tree, with values 9 and 15 respectively. Return 24.

The original problem is here.

My Solution

I solve this problem in C++, as below:

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/**
 * 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 res = 0;
    int sumOfLeftLeaves(TreeNode* root) {
        if (root == NULL) 
            return 0;
        if (isLeaf(root))
            return 0;
        sum(root);
        return res;

    }
    void sum(TreeNode* node) {
        if (node == NULL) 
            return;
        if (isLeaf(node->left))
             res += node->left->val;
        else{
            sum(node->left);
        }
        sum(node->right);
    }
    
    bool isLeaf(TreeNode* node) {
        if (node == NULL)
            return false;
        if(node->left == NULL && node->right == NULL)
            return true;
        return false;
    }
};

Note

To solve the problem, traverse the tree recursively, if the node is left leaf add the value to the result.