404. 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.

 

思路参考这里

方法一：递归

/**
 * 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 sumOfLeftLeaves(TreeNode* root) {
        if(!root||(!root->left&&!root->right)){
            return 0;
        }
        if(root->left&&!root->left->left&&!root->left->right){
            return root->left->val+sumOfLeftLeaves(root->right);
        }
        return sumOfLeftLeaves(root->left)+sumOfLeftLeaves(root->right);
    }
};

方法二：使用队列

/**
 * 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 sumOfLeftLeaves(TreeNode* root) {
        if(!root||(!root->left&&!root->right)){
            return 0;
        }
        queue<TreeNode*>q;
        q.push(root);
        int ans=0;
        while(!q.empty()){
            TreeNode* head=q.front();
            q.pop();
            if(head->left&&!head->left->left&&!head->left->right){
                ans=ans+head->left->val;
            }
            if(head->left){
                q.push(head->left);
            }
            if(head->right){
                q.push(head->right);
            }
        }
        return ans;
    }
};