【两次过】Lintcode 1254. Sum of Left Leaves

Find the sum of all left leaves in a given binary tree.

样例

    3
   / \
  9  20
    /  \
   15   7

There are two left leaves in the binary tree, with values 9 and 15 respectively. Return 24.

解题思路：

    递归。如何判断一个节点是左叶子节点（解决这个问题的核心）：对于一个节点root，root.left不为空，且 root.left.letf、root.left.right为空，则root.left这个节点是左叶子节点。

/**
 * Definition of TreeNode:
 * class TreeNode {
 * public:
 *     int val;
 *     TreeNode *left, *right;
 *     TreeNode(int val) {
 *         this->val = val;
 *         this->left = this->right = NULL;
 *     }
 * }
 */

class Solution {
public:
    /**
     * @param root: t
     * @return: the sum of all left leaves
     */
    int sumOfLeftLeaves(TreeNode * root) 
    {
        // Write your code here
        if(root == NULL)
            return 0;
        
        if(root->left != NULL && root->left->left == NULL && root->left->right == NULL)//若当前节点有左叶子节点
            return root->left->val + sumOfLeftLeaves(root->right);
        
        return sumOfLeftLeaves(root->left) + sumOfLeftLeaves(root->right);
    }
};