leetcode404-左叶子之和。没写出来，还要再看看

404. 左叶子之和

 

/**
 * 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) {
        if(root==nullptr) return 0;
        if(root->left==nullptr&&root->right==nullptr) return 0;
        int leftSum,rightSum;
        leftSum = sumOfLeftLeaves(root->left);
        if(root->left&&root->left->left==nullptr&&root->left->right==nullptr)
        {
            leftSum = root->left->val;
        }
        //leftSum = sumOfLeftLeaves(root->left);这一段不能放在这里，即不能和上一个if判断互换位置。如果当前节点的左孩子是叶子节点，这一段就会让leftSum直接变为0，从而出错
        rightSum = sumOfLeftLeaves(root->right);
        return leftSum+rightSum;
    }
};