[leetcode-513-Find Bottom Left Tree Value]

Given a binary tree, find the leftmost value in the last row of the tree.
Example 1:
Input:
  2
 / \
1 3

Output:
1
Example 2:
Input:
      1
     / \
    2 3
   / / \
  4 5 6
 /
7
Output:
7

思路：

层次遍历。每一层保存第一个数即可。

int findBottomLeftValue(TreeNode* root)
     {
         queue<TreeNode*>que;
         if (root!=NULL)que.push(root);        
         int ret = -1;
         while (!que.empty())
         {
             int size = que.size();             
             for (int i = 0; i < size;i++)//一层
             {
                 TreeNode* temp = que.front();
                 que.pop();
                 if (i ==0)ret = temp->val;
                 if (temp->left != NULL)que.push(temp->left);
                 if (temp->right != NULL)que.push(temp->right);
             }
         }
         return ret;
     }