404. 左叶子之和

当当前结点的左节点不为空，左节点的左节点和右节点为空，说明是叶子结点

一、递归

class Solution {
public:
    int sumOfLeftLeaves(TreeNode* root) {
        //终止条件
        if(root==NULL)return 0;
        //单层递归
        int leftnums = sumOfLeftLeaves(root->left);//左
        int rightnums = sumOfLeftLeaves(root->right);//右
        //中
        int midnums = 0;
        if(root->left && !root->left->left && !root->left->right)
            midnums+=root->left->val;
        return leftnums+rightnums+midnums;
    }
};

二、迭代

 class Solution {
public:
    int sumOfLeftLeaves(TreeNode* root) {
        if(root==NULL)return 0;
        stack<TreeNode*> st;
        int sum=0;
        st.push(root);
        while(!st.empty())
        {
            TreeNode* cur = st.top();
            st.pop();
            if(cur->left&& !cur->left->left && !cur->left->right)
                sum+=cur->left->val;
            if(cur->left)st.push(cur->left);
            if(cur->right)st.push(cur->right);
        }
        return sum;
    }
};