剑指Offer之Java算法习题精讲二叉树专项训练

题目一

 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);
     }
}

题目二

 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);
 
    }
}

题目三

 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);
    }
}

题目四

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;
    }
}

题目五

 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;
    }
}

题目六

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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