劍指Offer之Java算法習題精講二叉樹專項訓練

更新時間：2022年03月22日 09:26:43 作者：明天一定.

跟著思路走，之后從簡單題入手，反復去看，做過之后可能會忘記，之后再做一次，記不住就反復做，反復尋求思路和規(guī)律，慢慢積累就會發(fā)現(xiàn)質的變化

題目一

二叉樹題——查找二叉樹

根據(jù)給定的二叉樹根節(jié)點和指定條件查找其中指定元素

具體題目如下

解法

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    int i = 0;
    int res = 0;
    public int kthSmallest(TreeNode root, int k) {
        method(root,k);
        return res;
    }
     public void method(TreeNode root, int k){
        if(root==null) return ;
        method(root.left,k);
        i++;
        if(i==k){
            res = root.val;
            return ;
        }
        method(root.right,k);
     }
}

題目二

二叉樹題——轉換累加樹

根據(jù)給定的二叉樹根節(jié)點按指定條件轉換為累加樹

具體題目如下

解法

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    int sum = 0;
    public TreeNode convertBST(TreeNode root) {
        method(root);
        return root;
    }
    public void method(TreeNode root) {
        if(root==null){
            return;
        }
        method(root.right);
        sum+=root.val;
        root.val = sum;
        method(root.left);
 
    }
}

題目三

二叉樹題——驗證二叉樹

根據(jù)給定的二叉樹根節(jié)點判斷它是不是有效的二叉搜索樹

具體題目如下

解法

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public boolean isValidBST(TreeNode root) {
        return method(root,null,null);
    }
    public boolean method(TreeNode root,TreeNode min,TreeNode max){
        if(root==null) return true;
        if(min!=null&&root.val<=min.val) return false;
        if(max!=null&&root.val>=max.val) return false;
        return method(root.left,min,root)&&method(root.right,root,max);
    }
}

題目四

二叉樹題——搜索二叉樹

根據(jù)給定的二叉樹根節(jié)點查找其中指定的數(shù)值

具體題目如下

解法

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public TreeNode searchBST(TreeNode root, int val) {
        if(root==null) return null;
        if(root.val==val) return root;
        if(root.val>=val){
            return searchBST(root.left,val);
        }
        if(root.val<val){
            return searchBST(root.right,val);
        }
        return null;
    }
}

題目五

二叉樹題——二叉樹操作

根據(jù)給定的二叉樹根節(jié)點將指定的數(shù)值插入二叉樹

具體題目如下

解法

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public TreeNode insertIntoBST(TreeNode root, int val) {
        return  method(root,val);
    }
    public TreeNode method(TreeNode root, int val){
        if(root==null) return new TreeNode(val);
        if (root.val < val) 
            root.right = insertIntoBST(root.right, val);
        if (root.val > val) 
            root.left = insertIntoBST(root.left, val);
        return root;
    }
}

題目六

二叉樹題——二叉樹操作

根據(jù)給定的二叉樹根節(jié)點在不改變二叉樹性質的條件下刪除其中指定的數(shù)值對應的節(jié)點

具體題目如下

算法

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public TreeNode deleteNode(TreeNode root, int key) {
        if (root == null) return null;
        if (root.val == key){
            if (root.left == null) return root.right;
            if (root.right == null) return root.left;
            TreeNode minNode = getMin(root.right);
            root.right = deleteNode(root.right, minNode.val);
            minNode.left = root.left;
            minNode.right = root.right;
            root = minNode;
        }else if(root.val>key){
            root.left = deleteNode(root.left,key);
        }else{
            root.right = deleteNode(root.right,key);
        }
        return root;
    }
    TreeNode getMin(TreeNode node) {
        while (node.left != null) node = node.left;
        return node;
    }   
}

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