面试29: 数组中出现次数超过一半的数字

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一. 题目

数组中有一个数字出现的次数超过数组长度的一半,请找出这个数字.例如输入一个长度为9的数组{1,2,3,2,2,2,5,4,2}.由于数字2在数组中出现了5次,超过数组长度的一半,因此输出2.

代码请到我的代码库中下载 Point2Offer

二. 代码

package com;/** * 剑指offer: 数组中出现次数超过一半的数字 * 方法:1.排序,中位数(改变了数组);2.数字排列规律(过半,不管怎么消,都会剩下1) * 测试用例:功能测试(数组中存在,不存在这样的数字) * 特殊输入测试(数组只有一个数字,数组为空) * @author dingding * Date:2017-6-16 14:00 * Declaration: All Rights Reserved! */public class MoreThanHalfNumber {    static boolean inputInvalid = false;    public static void main(String[] args) {        test1();        test2();    }    //方法2:统计规律(更简单)    private static int findMoreThanHalfNum_2(int[] arr,int length){        if (checkInvalidArray(arr, length)) {            return 0;   //TODO:处理过半为0        }        int result = arr[0];        int times = 1;        for (int i=0;i<length;i++){            if (times == 0) {                result = arr[i];                times=1;            }else if (arr[i]==result) {                times++;            }else{                times--;            }        }        if (!checkMoreThanHalf(arr, length, result)) {            result = 0;        }        return result;    }    //方法1:快排    private static int findMoreThanHalfNum_1(int[] arr,int length){        if (checkInvalidArray(arr,length)) {            return 0;        }        int middle = length>>1;   //取中间的数        int start = 0;        int end = length-1;        int index = partition(arr,length,start,end);        while (index!=middle){            if (index>middle) {                end = index-1;                index = partition(arr, length, start, end);            }else {                start = index+1;                index = partition(arr, length, start, end);            }        }        int result = arr[middle];  //      System.out.println(index+"==?"+middle);        if (!checkMoreThanHalf(arr,length,result)) {            return 0;        }        return result;    }    //获取数组分片位置    private static int partition(int[] arr, int length, int start, int end) {        int i = start-1;        int x = arr[end];        for (int j=start;j<end;j++){            if (arr[j]<=x) {                i++;                swap(arr,i,j);            }        }        swap(arr,i+1,end);        return i+1;    }    //交换位置    private static void swap(int[] arr, int i, int j) {        int temp;        temp = arr[i];        arr[i] = arr[j];        arr[j] =temp;    }    //检查是否过半    private static boolean checkMoreThanHalf(int[] arr, int length, int result) {        int times = 0;        for (int i=0;i<length;i++){            if (arr[i]==result) {                times++;            }        }        boolean isMoreThanHalf = true;        if (times*2 <= length) {            isMoreThanHalf = false;            inputInvalid = true;        }        return isMoreThanHalf;    }    //判断输入的数组是否有效    private static boolean checkInvalidArray(int[] arr, int length) {        if (arr == null||length<=0) {            inputInvalid = true;        }        return inputInvalid;    }    /*======================测试用例================*/    ////方法1结果    private static void test1() {        System.out.println("方法1:  ");        int[] numbers = {1};        int result = findMoreThanHalfNum_1(numbers,numbers.length);        if (result==0) {            System.out.println("Not Found!");        }else {            System.out.println(result);        }    }    //方法2结果    private static void test2(){        System.out.println("方法1:  ");        int[] numbers = {1, 2, 3, 2, 2, 2, 5, 4, 2};        int result = findMoreThanHalfNum_2(numbers,numbers.length);        if (result==0) {            System.out.println("Not Found!");        }else {            System.out.println(result);        }    }}

有不妥当之处,麻烦告知:D

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