这种写法，效率比较高，每一个节点只访问一次。

struct TreeNode {
	int val;
	struct TreeNode *left;
	struct TreeNode *right;
	TreeNode(int x) :
			val(x), left(NULL), right(NULL) {
	}
};

class Solution {
public:
    bool IsBalanced_Solution(TreeNode* pRoot) {
		
		int depth = 0;
		return isBalanceTree(pRoot,depth);
    }

	/* 后序遍历，这样每一个节点，只访问一次 */
	bool isBalanceTree(TreeNode* pRoot, int& depth)
	{
		if(pRoot == NULL)
		{
			depth = 0;
			return true;
		}

		int left = 0;
		int right = 0;
		if(isBalanceTree(pRoot->left,left) && isBalanceTree(pRoot->right,right))
		{
			int deff = left - right;
			if(deff <= 1 && deff >= -1)
			{
				depth = left > right ? left : right;
				depth += 1;
				return true;
			}			
		}

		return false;
	}
};