题目：

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代码(首刷看解析）：

class Solution {
public:int sumOfLeftLeaves(TreeNode* root) {int res = 0;if (root == nullptr) return res;queue<TreeNode*> que;TreeNode* cur = root;que.push(cur);int size = 0;while (!que.empty()) {size = que.size();while (size--) {TreeNode* node = que.front();que.pop();if (node->left) {que.push(node->left);if (node->left->left == nullptr && node->left->right == nullptr) {res += node->left->val;}  }if (node->right) que.push(node->right);}}return res;}
};