657. Judge Route Circle
     思路：經(jīng)歷上下左右移動(dòng)，判斷是否能回到原點(diǎn)。即判斷左移的步數(shù)是否等于右移的步數(shù)；上移的步數(shù)是否等于下移的步數(shù)

class Solution(object):
    def judgeCircle(self, moves):
        """
        :type moves: str
        :rtype: bool
        """
        if moves.count('U') == moves.count('D') and moves.count('L') == moves.count('R'):
            return True
        return False

617. Merge Two Binary Trees
     思路：將兩棵樹上的結(jié)點(diǎn)值相加，用遞歸的方法。如果子節(jié)點(diǎn)空，循環(huán)就結(jié)束了。

class Solution(object):
    def mergeTrees(self, t1, t2):
        """
        :type t1: TreeNode
        :type t2: TreeNode
        :rtype: TreeNode
        """
        if not t1 and not t2: return None
        ans = TreeNode((t1.val if t1 else 0) + (t2.val if t2 else 0))
        ans.left = self.mergeTrees(t1 and t1.left, t2 and t2.left)
        ans.right = self.mergeTrees(t1 and t1.right, t2 and t2.right)
        return ans