104. Maximum Depth of Binary Tree

這題很簡(jiǎn)單，只要理解樹(shù)的深度 = max{ 左子樹(shù)深度, 右子樹(shù)深度} + 1就可以了。還有就是要理解遞歸。

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public int maxDepth(TreeNode root) {
        if (root == null) {
            return 0;
        }
        int leftDepth = maxDepth(root.left);
        int rightDepth = maxDepth(root.right);
        return Math.max(leftDepth, rightDepth) + 1;
    }
}

111. Minimum Depth of Binary Tree

這題跟上邊那題有點(diǎn)微妙的區(qū)別，如果試圖套用樹(shù)的深度 = min{左子樹(shù)深度， 右子樹(shù)深度} + 1，那么有幾個(gè)測(cè)試用例是過(guò)不了的。

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public int minDepth(TreeNode root) {
        if (root == null) {
            return 0;
        }
        int leftDepth = minDepth(root.left);
        int rightDepth = minDepth(root.right);
        if (leftDepth == 0 || rightDepth == 0) {
            return leftDepth == 0 ? rightDepth + 1 : leftDepth + 1;
        } else {
            return Math.min(leftDepth, rightDepth) + 1;
        }
    }
}