Tree, Min/Max/Balanced Depth

• 一个树depth的定义是，最深处node的depth，所以要取max.

Maximum Depth of Binary Tree

public class Solution {
    public int maxDepth(TreeNode root) {
        if (root == null) {
            return 0;
        }

        int left = maxDepth(root.left);
        int right = maxDepth(root.right);

        return Math.max(left, right) + 1;
    }
}

Minimum Depth of Binary Tree

一个意思，不过注意拐弯处如果缺了一个child并不代表valid path， 因为这个节点不是leaf node。

public class Solution {
    public int minDepth(TreeNode root) {
        if (root == null) {
            return 0;
        } 

        int left = minDepth(root.left);
        int right = minDepth(root.right);

        if (root.left != null && root.right != null) {
            return Math.min(left, right) + 1;
        } else if (root.left == null) {
            return 1 + right;
        } else {
            return 1 + left;
        }
    }
}

Balanced Binary Tree

已经不平衡的地方直接return -1，避免去进一步做不必要的递归。

public class Solution {
    public boolean isBalanced(TreeNode root) {
        return helper(root) != -1;
    }

    private int helper(TreeNode root) {
        if (root == null) {
            return 0;
        }

        int left = helper(root.left);
        int right = helper(root.right);

        if (left == -1 || right == -1) {
            return -1;
        }

        if (Math.abs(left - right) > 1) {
            return -1;
        }

        return Math.max(left, right) + 1;
    }
}