Given a binary tree, determine if it is height-balanced.
For this problem, a height-balanced binary tree is defined as a binary tree in which the depth of the two subtrees of every node never differ by more than 1.
分别求左右子树的高度，如果高度之差大于1，则不是平衡树
而如果子树不是平衡树，结果也一定不是平衡树，因此再求左右子树的同时，顺便判断是否平衡树，能够提高速度
bool isBalanced(TreeNode *root) {
bool unbalenced = false;
getHeight(unbalenced, root);
return !unbalenced;
}
int getHeight(bool &unbalenced, TreeNode *root) {
if (root == NULL || unbalenced)
return -1;
int left = getHeight(unbalenced, root->left);
int right = getHeight(unbalenced, root->right);
if (abs(left, right) > 1) {
unbalenced = true;
}
return 1 + (left > right ? left : right);
}