Maximum Depth of Binary Tree

我自己写的divide&conquer

/**
 * Definition of TreeNode:
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left, right;
 *     public TreeNode(int val) {
 *         this.val = val;
 *         this.left = this.right = null;
 *     }
 * }
 */
public class Solution {
    /**
     * @param root: The root of binary tree.
     * @return: An integer.
     */
    public int maxDepth(TreeNode root) {
        // write your code here
        if (root == null) {
            return 0;  //?
        }
        int result = divide(root);
        return result;
    }
    private int divide(TreeNode root) {
        if (root == null) {
            return 0;
        }
        int re1 = divide(root.left);
        int re2 = divide(root.right);
        if (re1 >= re2) {
            return re1 + 1;
        } else {
            return re2 +1;
        }

    }
}

九章

// Version 1: Divide Conquer
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;
    }
}

递归老写错，摊手╮(╯-╰)╭

正确的：我抄的答案

/**
* Definition of TreeNode:
* public class TreeNode {
* public int val;
* public TreeNode left, right;
* public TreeNode(int val) {
* this.val = val;
* this.left = this.right = null;
* }
* }
*/

public class Solution {
/**
* @param root: The root of binary tree.
* @return: An integer.
*/
    private int result;
    public int maxDepth(TreeNode root) {
    // write your code here
        result = 0;   //这里不能写int result 不能重新定义
        traverse(root, 1);//传入1不是0
        return result;
    }
    private void traverse(TreeNode root, int curr) {
        if (root == null) {
        return ;
        }
        
        if (curr > result) { //先写这个
            result = curr;
        }
        traverse(root.left, curr + 1);
        traverse(root.right, curr + 1);
    }
}

九章

// version 2: Traverse
/**
 * Definition of TreeNode:
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left, right;
 *     public TreeNode(int val) {
 *         this.val = val;
 *         this.left = this.right = null;
 *     }
 * }
 */
public class Solution {
    /**
     * @param root: The root of binary tree.
     * @return: An integer.
     */
    private int depth;
    
    public int maxDepth(TreeNode root) {
        depth = 0;
        helper(root, 1);
        
        return depth;
    }
    
    private void helper(TreeNode node, int curtDepth) {
        if (node == null) {
            return;
        }
        
        if (curtDepth > depth) {
            depth = curtDepth;
        }
        
        helper(node.left, curtDepth + 1);
        helper(node.right, curtDepth + 1);
    }
}

错的

/**
* Definition of TreeNode:
* public class TreeNode {
* public int val;
* public TreeNode left, right;
* public TreeNode(int val) {
* this.val = val;
* this.left = this.right = null;
* }
* }
*/
public class Solution {
/**
* @param root: The root of binary tree.
* @return: An integer.
*/
public int maxDepth(TreeNode root) {
// write your code here
if (root == null) {
return 0; //?
}
int result = 0;
traverse(root, result);
return result;
}
private void traverse(TreeNode root, int result) {
if (root == null) {
return;
}
int re1 = 0;
int re2 = 0;
traverse(root.left, re1);
traverse(root.right, re2);
if (re1 >= re2) {
result = re1 + 1;
} else {
result = re2 +1;
}

}
}