几个面试经典算法题Java解答
题目一：
public class testClockwiseOutput { //顺时针打印一个矩阵 @Test public void test(){ int[][] num = new int[100][100]; int n = 4; int count =1; for(int i=0;i<n;i++){ for(int j =0;j<n;j++){ num[i][j]=count++; } } output(num,0,n-1); } public void output(int[][] num,int start,int end){ if(start>=end || end<=0)return; for(int i=start;i<=end;i++){ System.out.println(num[start][i]); } for(int i=start+1;i<=end;i++){ System.out.println(num[i][end]); } for(int i=end-1;i>=start;i--){ System.out.println(num[end][i]); } for(int i=end-1;i>start;i--){ System.out.println(num[i][start]); } output(num,start+1,end-1); } }
题目二：
给出一个排序好的数组和一个数，求数组中连续元素的和等于所给数的子数组
//给出一个排序好的数组和一个数，求数组中连续元素的和等于所给数的子数组 @Test public void test(){ int[] num = {1,2,2,3,4,5,6,7,8,9}; int sum = 7; findSum(num,sum); } public void findSum(int[] num,int sum){ int left=0; int right=0; for(int i=0;i<num.length;i++){ int curSum = 0; left = i; right = i; while(curSum<sum){ curSum += num[right++]; } if(curSum==sum){ for(int j=left;j<right;j++){ System.out.print(num[j]+" "); } System.out.println(); } } }
题目三：
//字符数组组成的所有字符串 @Test public void test(){ //char[] cs = {'a','b','c','d','e'}; char[] cs = {'a','b','c'}; int length = cs.length; recursionSwap(cs,0,length); } public void swap(char[] cs,int index1,int index2){ char temp = cs[index1]; cs[index1]=cs[index2]; cs[index2]=temp; } public void recursionSwap(char[] cs,int start,int length){ if(start>=length-1){ print(cs); return; } for(int i=start;i<length;i++){ swap(cs,start,i); recursionSwap(cs,start+1,length); swap(cs,start,i); } } public void print(char[] cs){ for(int i=0;i<cs.length;i++){ System.out.print(cs[i]); } System.out.println(); }
题目四：
//数组组成的最小数 @Test public void test(){ int[] num={1,5,9,13,442,44,6,21,211}; qsort(num,0,num.length-1); System.out.println(Arrays.toString(num)); } public void qsort(int[] num,int left,int right){ if(left<right){ int partition = partition(num,left,right); qsort(num,left,partition-1); qsort(num,partition+1,right); } } public int partition(int[] num,int left,int right){ int partition = num[left]; while(left<right){ while((num[right]==partition || isMBigerThanN(num,num[right],partition)) && left<right){ right--; } swap(num,left,right); while((num[left]==partition || isMBigerThanN(num,partition,num[left])) && left<right){ left++; } swap(num,left,right); } return left; } public void swap(int[] num,int m,int n){ int temp = num[m]; num[m]=num[n]; num[n]=temp; } public boolean isMBigerThanN(int[] num,int m,int n){ String num1 = String.valueOf(m); String num2 = String.valueOf(n); int temp1 = Integer.parseInt(num1+num2); int temp2 = Integer.parseInt(num2+num1); if(temp1>temp2){ return true; } else{ return false; } }
题目五：
//子数组最大和 @Test public void test(){ int[] num = {1,-2,3,10,-4,7,2,-5}; //int[] num = {1,-2,3,10,-4,10,2,-5}; System.out.println(maxSum(num)); } public int maxSum(int[] num){ int curSum = 0; int curMaxSum = -99999999; int start = 0; int end = 0; for(int i=0;i<num.length;i++){ if(curSum<=0){ curSum = num[i]; start = i; } else{ curSum += num[i]; } if(curSum>curMaxSum){ curMaxSum = curSum; end = i; } } for(int i = start;i<=end;i++){ System.out.println(num[i]); } return curMaxSum; }
题目六：
public class testMinStack { //自定义栈，min函数得到当前最小值 @Test public void test(){ MinStack ms = new MinStack(); ms.push(5); System.out.println(ms.min()); ms.push(6); ms.push(2); ms.push(1); System.out.println(ms.min()); ms.pop(); System.out.println(ms.min()); ms.pop(); System.out.println(ms.min()); } } class MinStack{ private Stack<Integer> minStack = new Stack<Integer>(); private Stack<Integer> stack = new Stack<Integer>(); public int pop(){ minStack.pop(); return stack.pop(); } public void push(int num){ if(minStack.size()<=0){ minStack.push(num); return; } Integer min = minStack.lastElement(); if(num<min){ minStack.push(num); } else{ minStack.push(min); } stack.push(num); } public int min(){ if(minStack.size()<=0){ return -1; } return minStack.lastElement(); } }
题目七：
//找出数组中出现次数大于一半的数 @Test public void test(){ int[] num = {1,2,2,2,2,2,2,4,2,4,6,4,2,6,8,2,7,7}; System.out.println(moreThanHaft(num)); } public int moreThanHaft(int[] num){ int result = -1; int times = 0; for(int i=0;i<num.length;i++){ if(times==0){ result = num[i]; times++; } else{ if(num[i]==result){ times++; } else{ times--; } } } return result; }
题目八：
//判断一个数组是否是另一个栈的出栈顺序 @Test public void test(){ int[] num = {1,2,3,4,5}; //int[] num1={1,2,3,5,4}; int[] num2={2,1,5,3,4}; Stack<Integer> s1 = new Stack<Integer>(); Stack<Integer> s2 = new Stack<Integer>(); for(int i=0;i<5;i++){ s2.push(num2[i]); } System.out.println(testOrder(num,s1,s2)); } public boolean testOrder(int[] num,Stack<Integer> s1,Stack<Integer> s2){ int length = num.length; for(int i=0;i<length;i++){ s1.push(num[i]); int s2Num = s2.lastElement(); if(s2Num==s1.lastElement().intValue()){ s1.pop(); s2.pop(); } } while(!s1.isEmpty()){ if(!s1.pop().equals(s2.pop())){ return false; } } return true; }
题目九：
//从扑克牌抽5张牌，0可以为任意数，判断是否是顺子 @Test public void test(){ int[] num = {0,1,5,3,2}; System.out.println(check(num)); } public boolean check(int[] num){ //0-13 int[] pai = new int[14]; for(int n : num){ pai[n]+=1; } qsort(num,0,num.length-1); int count = pai[0]; int start = 0; if(num[0]==0){ start=num[1]; } else{ start=num[0]; } for(int i = start;i<=start+5;i++){ if(pai[i]>1)return false; count += pai[i]; } if(count == 5)return true; else return false; } public void qsort(int[] num,int left,int right){ if(left<right){ int partition = partition(num,left,right); qsort(num,left,partition-1); qsort(num,partition+1,right); } } public int partition(int[] num,int left,int right){ int partition = num[left]; while(left<right){ while(left<right && num[right]>=partition){ right--; } swap(num,left,right); while(left<right && num[left]<=partition){ left++; } swap(num,left,right); } return left; } public void swap(int[] num,int m,int n){ int temp = num[m]; num[m]=num[n]; num[n]=temp; }
题目十：
//输出第k个丑数(因子只有2，3，5) @Test public void test(){ findUglyNum(8); } public void findUglyNum(int index){ int[] num = new int[index]; int next = 1; num[0]=1; int index2=0; int index3=0; int index5=0; while(next<index){ int num2 = num[index2]*2; int num3 = num[index3]*3; int num5 = num[index5]*5; num[next] = getSuitable(num2,num3,num5); while(num[index2]*2<=num[next]){ index2++; } while(num[index3]*3<=num[next]){ index3++; } while(num[index5]*5<=num[next]){ index5++; } next++; } System.out.println(num[index-1]); } public int getSuitable(int num2,int num3,int num5){ int s = num2; if(num3<s){ s = num3; } if(num5<s){ s = num5; } return s; }
查看21道真题和解析