剑指offer-数组中出现次数超过一半的数字

题目描述

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。如果不存在则输出0。

解法一：O（n）

package demo1;

import org.junit.Test;

public class Solution {
public int MoreThanHalfNum_Solution(int[] array) {
if (array == null || array.length <= 0) {
return 0;
}
int result = array[0];
int times = 1;
// 相同加一，不同减一
for (int i = 1; i < array.length; i++) {
if (times == 0) {
result = array[i];
times = 1;
} else if (result == array[i]) {
times++;
} else {
times--;
}
}
// 进一步验证是否超过总数一半
if (!checkMoreThanHalf(array, result)) {
result = 0;
}
System.out.println(result);
return result;
}

private boolean checkMoreThanHalf(int[] array, int result) {
int times = 0;
for (int i = 0; i < array.length; i++) {
if (array[i] == result) {
times++;
}
}
Boolean isMore = true;

if (times * 2 <= array.length) {
isMore = false;
}
return isMore;
}

@Test
public void test() {
int[] array = { 1, 2, 3, 2, 2, 2, 5, 4, 2 };
MoreThanHalfNum_Solution(array);
}
}

解法二：排序后，总多的数就是中位数（快排）

package demo1;

import org.junit.Test;

public class Solution {
public int MoreThanHalfNum_Solution(int[] array) {
if(array ==null || array.length <=0 ) {
return 0;
}
int middle = array.length >> 1;
quickSort(array, 0, array.length-1);
int result = array[middle];
if (!checkMoreThanHalf(array, result)) {
result = 0;
}
System.out.println(result);
return result;
}

private void quickSort(int[] array, int low, int high) {
// TODO Auto-generated method stub
int i = low, j = high, index;
if (low >= high)
return;
index = array[i];
while (i < j) {
while (i < j && array[j] >= index) {
j--;
}
if (i < j) {
array[i] = array[j];
i++;
}
while (i < j && array[i] < index) {
i++;
}
if (i < j) {
array[j] = array[i];
j--;
}
}
array[i] = index;
quickSort(array, low, i - 1);
quickSort(array, i + 1, high);
}

private boolean checkMoreThanHalf(int[] array, int result) {
int times = 0;
for (int i = 0; i < array.length; i++) {
if (array[i] == result) {
times++;
}
}
Boolean isMore = true;

if (times * 2 <= array.length) {
isMore = false;
}
return isMore;
}

@Test
public void test() {
int[] array = { 1, 2, 3, 2, 2, 2, 5, 4, 2 };
MoreThanHalfNum_Solution(array);
}
}