leetcode解题: Sum of Left Leaves (404)

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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.

解法1：Recursion O(N) Time

二叉树很自然的想到用递归的办法解决。这里的难点是要记录每一个节点是否为Left child/right child，为此我们需要有一个helper函数来记录。根据OA的结果，root是算作非左子树的。
C++

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/**
 * 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) {
        return helper(root, false);
    }
    int helper(TreeNode* root, bool isleft) {
        if (root == NULL) return 0;
        if (root->left == NULL && root->right == NULL) {
            return isleft? root->val: 0;
        }
        int left = helper(root->left, true);
        int right = helper(root->right, false);
        return left + right;
    }
};

Java

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