[LeetCode August Challange]Day24-Sum of Left Leaves

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.

---

Solution

Time complexity : O(n)
Space complexity : O(n)

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    int sumOfLeftLeaves(TreeNode* root) {
        sum_ = 0;
        preTraversal(root);
        return sum_;
    }
private:
    int sum_;
    void preTraversal(TreeNode* node) {
        if (!node) return;
        if (node->left && isLeaf(node->left))
            sum_ += node->left->val;
        
        preTraversal(node->left);
        preTraversal(node->right);
    }
    bool isLeaf(TreeNode* node) {
        return !node->left && !node->right;
    }
};

尋訪二元樹，前中後皆可，找到左子節點且為葉節點，將其值加入sum_中即可。