题目：
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.
思路：
1) 计算结点的高度，但这个有重复计算
package tree;
public class BalancedBinaryTree {
public boolean isBalanced(TreeNode root) {
if (root == null) return true;
int leftHeight = height(root.left);
int rightHeight = height(root.right);
return Math.abs(leftHeight - rightHeight) <= 1 &&
isBalanced(root.left) && isBalanced(root.right);
}
private int height(TreeNode root) {
if (root == null) return 0;
return Math.max(1 + height(root.left), 1 + height(root.right));
}
}
2) 不进行重复计算，但需要new 对象，反而也花时间
package tree;
public class BalancedBinaryTree {
class Entry{
public int height;
public boolean balanced;
}
public boolean isBalanced(TreeNode root) {
return checkTree(root).balanced;
}
private Entry checkTree(TreeNode root) {
Entry entry = new Entry();
if (root == null) {
entry.height = 0;
entry.balanced = true;
} else {
Entry left = checkTree(root.left);
Entry right = checkTree(root.right);
entry.height = Math.max(left.height, right.height) + 1;
entry.balanced = Math.abs(left.height - right.height) <= 1 && left.balanced && right.balanced;
}
return entry;
}
}