/**** Source : https://oj.leetcode.com/problems/maximum-subarray/** Created by lverpeng on 2017/7/18.** Find the contiguous subarray within an array (containing at least one number)* which has the largest sum.** For example, given the array [−2,1,−3,4,−1,2,1,−5,4],* the contiguous subarray [4,−1,2,1] has the largest sum = 6.** More practice:** If you have figured out the O(n) solution, try coding another solution using* the divide and conquer approach, which is more subtle.**/
public class MaxSubarray {/*** 找到所有子数组中最大的和** 数组从前向后遍历，针对每个数组元素有两个选择，要么加入已经存在子数组，如果该元素的值大于该元素和前面数组总和的和还要大，*          那就重新开始一个新的子数组，遍历完数组找到最大的和*** @param arr* @return*/public int maxSubarray (int[] arr) {int loopCount = 0;int[] sum = new int[arr.length];int max = 0;sum[0] = arr[0];for (int i = 1; i < arr.length; i++) {sum[i] = Math.max(arr[i], sum[i - 1] + arr[i]);max = Math.max(max, sum[i]);loopCount ++;}System.out.println("maxSubarray-->" + loopCount);return max;}public int maxSubarray1 (int[] arr) {int loopCount = 0;// 只记录上一个和int sum = 0;int max = 0;for (int i = 1; i < arr.length; i++) {if (sum < 0) {sum = 0;}sum += arr[i];max = Math.max(max, sum);loopCount ++;}System.out.println("maxSubarray1-->" + loopCount);return max;}/*** 使用分治法** @param arr* @return*/int loopCount1 = 0;public int maxSubarray2 (int[] arr) {loopCount1 = 0;return divide(arr, 0, arr.length - 1);}private int divide (int[] arr, int low, int high) {if (low == high) {return arr[low];}if (low == high - 1) {return Math.max(arr[low] + arr[high], Math.max(arr[low], arr[high]));}int mid = (low + high) / 2;int lmax = divide(arr, low, mid - 1);int rmax = divide(arr, mid + 1, high);int mmax = arr[mid];int temp = mmax;for (int i = mid - 1; i > 0; i--) {temp += arr[i];if (mmax < temp) {mmax = temp;}loopCount1 ++;}temp = mmax;for (int i = mid + 1; i < high; i++) {temp += arr[i];if (temp > mmax) {mmax = temp;}loopCount1 ++;}System.out.println("maxSubarray2-->" + loopCount1);return Math.max(mmax, Math.max(lmax, rmax));}public static void main(String[] args) {MaxSubarray maxSubarray = new MaxSubarray();int[] arr = new int[]{-2,1,-3,4,-1,2,1,-5,4};System.out.println(maxSubarray.maxSubarray(arr));System.out.println(maxSubarray.maxSubarray1(arr));System.out.println(maxSubarray.maxSubarray2(arr));int[] arr1 = new int[]{-2,1,-3,4,-1,2,1,-5,4,-2,1,-3,4,-1,2,1,-5,4,-2,1,-3,4,-1,2,1,-5,4,-2,1,-3,4,-1,2,1,-5,4};System.out.println(maxSubarray.maxSubarray(arr1));System.out.println(maxSubarray.maxSubarray1(arr1));System.out.println(maxSubarray.maxSubarray2(arr1));}
}