劍指offer java實現合集（6）第26~30題

26.二叉搜索樹與雙向鏈表

模仿中序遍歷的思路，但是這次是從右向左進行。

public class Solution {
    TreeNode temp = null;
    public TreeNode Convert(TreeNode pRootOfTree) {
      
       if(pRootOfTree==null){
           return null;
       }
       Convert(pRootOfTree.right);
       if(temp==null){
           temp = pRootOfTree;
       }else{
           temp.left = pRootOfTree;
           pRootOfTree.right = temp;
           temp = pRootOfTree;
       }
        Convert(pRootOfTree.left);
        return temp;
    }
}

27.字符串的排列

 

import java.util.ArrayList;
import java.util.*;
public class Solution {
    ArrayList<String> res = new ArrayList();
    public ArrayList<String> Permutation(String str) {
        ArrayList<String> res = new ArrayList();
        int len = str.length();
        if(len==0){
            return res;
        }
        char []cha = str.toCharArray();
        getres(cha,0,len,res);
        Collections.sort(res);
        return res;
    }
    public ArrayList<String> getres(char[] cha,int i ,int len,ArrayList list){
        if(i==len-1){
            String temp = String.valueOf(cha);
            if(!list.contains(temp)){
                list.add(temp);
            }
        }else{
            for(int j = i;j<len;j++){
                swap(cha,i,j);
                getres(cha,i+1,len,list);
                swap(cha,i,j);
            }
        }
        return list;
    }
    public void swap(char cha[],int i ,int j){
        char x = cha[i];
        cha[i] = cha[j];
        cha[j] = x;
    }
}

28.數組中超過出現次數超過一半的數字

排序之後，如存在這樣的數，取中位數就是答案，然後驗證出現的次數是否符合超過半數的設定。

import java.util.*;
public class Solution {
    public int MoreThanHalfNum_Solution(int [] array) {
        Arrays.sort(array);
        int len = array.length;
        int mid = len/2;
        int res = array[mid];
        int count = 0;
        for(int x :array){
            if(x==res){
                count++;
            }
        }
        if(count>mid){
            return res;
        }else{
            return 0;
        }
    }
}

29.最小的K個數

考察的應該是堆排序，後續會添加上。

import java.util.*;
public class Solution {
    public ArrayList<Integer> GetLeastNumbers_Solution(int [] input, int k) {
        ArrayList<Integer> arr = new ArrayList();
        int len = input.length;
        if(len==0||k>len){
            return arr;
        }
        Arrays.sort(input);
        int i =0;
        while(k>0){
            arr.add(input[i]);
            i++;
            k--;
        }
        return arr;
    }
}

30.連續子數組的最大和

能讓整體的和更大的方式，就是之前數的和爲整數，如果爲負數，就拋棄之前所有的數，以當前數作爲起點。

public class Solution {
    public int FindGreatestSumOfSubArray(int[] array) {
        int res = array[0];
        int temp = array[0];
        for(int i =1;i<array.length;i++){
            if(temp>=0){
                temp = temp+array[i];
            }else{
                temp = array[i];
            }
            if(temp>res){
                res = temp;
            }
        }
        return res;
    }
}