剑指Offer之Java算法习题精讲数组与二叉树

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 {
    public TreeNode sortedArrayToBST(int[] nums) {
        return method(nums,0,nums.length-1);
    }
    public TreeNode method(int[] nums,int lf,int rg){
        if(lf>rg){
            return null;
        }
        int mid = lf+(rg-lf)/2;
        TreeNode root = new TreeNode(nums[mid]);
        root.left = method(nums,lf,mid-1);
        root.right = method(nums,mid+1,rg);
        return root;        
    }
}

题目二

 解法

class Solution {
    public boolean containsDuplicate(int[] nums) {
       HashSet<Integer> set = new HashSet<Integer>();
       for(int i = 0;i<nums.length;i++){
           if(!set.add(nums[i])){
               return true;
           }
           set.add(nums[i]);
       }
       return false;
    }
}

题目三

 解法

class Solution {
    public boolean containsNearbyDuplicate(int[] nums, int k) {
        Map<Integer, Integer> map = new HashMap<Integer, Integer>();
        int length = nums.length;
        for (int i = 0; i < length; i++) {
            int num = nums[i];
            if (map.containsKey(num) && i - map.get(num) <= k) {
                return true;
            }
            map.put(num, i);
        }
        return false;
    }
}

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