algorithm-class/公开课/class026/Code01_SubArrayMaxSum.java

package class026;

import java.util.ArrayList;
import java.util.List;

public class Code01_SubArrayMaxSum {

	public static int test(int[] arr) {
		if (arr == null || arr.length == 0) {
			return 0;
		}
		int N = arr.length;
		int max = Integer.MIN_VALUE;
		for (int L = 0; L < N; L++) {
			for (int R = L; R < N; R++) {
				// arr[L...R]
				int sum = 0;
				for (int i = L; i <= R; i++) {
					sum += arr[i];
				}
				max = Math.max(max, sum);
			}
		}
		return max;
	}

	public static int dp1(int[] arr) {
		if (arr == null || arr.length == 0) {
			return 0;
		}
		int[] dp = new int[arr.length];
		// dp[i]  子数组必须以i位置结尾的情况下，能得到的最大累加和
		dp[0] = arr[0];
		for (int i = 1; i < arr.length; i++) {
			int p1 = arr[i];
			int p2 = dp[i - 1] + arr[i];
			dp[i] = Math.max(p1, p2);
		}
		int max = Integer.MIN_VALUE;
		for (int i = 0; i < dp.length; i++) {
			max = Math.max(max, dp[i]);
		}
		return max;
	}
	
	public static int dp2(int[] arr) {
		if (arr == null || arr.length == 0) {
			return 0;
		}
		int preDp = arr[0];
		int max = preDp;
		for (int i = 1; i < arr.length; i++) {
			int p1 = arr[i];
			int p2 = preDp + arr[i];
			int dp = Math.max(p1, p2);
			max = Math.max(max, dp);
			preDp = dp;
		}
		return max;
	}

	public static int maxSum(int[] arr) {
		if (arr == null || arr.length == 0) {
			return 0;
		}
		int cur = 0;
		int max = Integer.MIN_VALUE;
		for (int i = 0; i < arr.length; i++) {
			cur += arr[i];
			max = Math.max(max, cur);
			cur = cur < 0 ? 0 : cur;
		}
		return max;
	}

	public static int maxSum1(int[] arr) {
		if (arr == null || arr.length == 0) {
			return 0;
		}
		// 0结尾时候的答案
		int pre = arr[0];
		int max = arr[0];
		for (int i = 1; i < arr.length; i++) {
			// i结尾时候的答案
			pre = arr[i] + (pre > 0 ? pre : 0);
			max = Math.max(max, pre);
		}
		return max;
	}

	public static List<List<Integer>> maxSum2(int[] arr) {
		List<List<Integer>> ans = new ArrayList<>();
		if (arr == null || arr.length == 0) {
			return ans;
		}
		int L = 0;
		int maxLen = 0;
		int maxSum = Integer.MIN_VALUE;
		int cur = 0;
		for (int i = 0; i < arr.length; i++) {
			// L...i sum
			cur += arr[i];
			if (cur == maxSum && (i - L + 1) == maxLen) {
				List<Integer> curAns = new ArrayList<>();
				curAns.add(L);
				curAns.add(i);
				ans.add(curAns);
			}
			if (cur > maxSum || (cur == maxSum && (i - L + 1) > maxLen)) {
				ans.clear();
				List<Integer> curAns = new ArrayList<>();
				curAns.add(L);
				curAns.add(i);
				ans.add(curAns);
				maxLen = i - L + 1;
			}
			maxSum = Math.max(maxSum, cur);
			if (cur < 0) {
				cur = 0;
				L = i + 1;
			}
		}
		return ans;
	}

	public static int[] generateArray(int N, int V) {
		int n = (int) (Math.random() * N) + 1;
		int[] arr = new int[n];
		for (int i = 0; i < n; i++) {
			arr[i] = (int) (Math.random() * V) - (int) (Math.random() * V);
		}
		return arr;
	}

	public static void main(String[] args) {
		int N = 100;
		int V = 100;
		int testTime = 10000;
		System.out.println("test begin");
		for (int i = 0; i < testTime; i++) {
			int[] arr = generateArray(N, V);
			int ans1 = test(arr);
			int ans2 = dp1(arr);
			int ans3 = dp2(arr);
			if (ans1 != ans2 || ans1 != ans3) {
				System.out.println("Oops!");
			}
		}
		System.out.println("test finish");

		int[] test = { 2, 2, 1, -9, 2, 3, -9, 6, -9, 2, 2, 2, -9, 2, 2, 2, -9, 1, 4, 1 };

		List<List<Integer>> ans = maxSum2(test);

		for (List<Integer> cur : ans) {
			System.out.println("start : " + cur.get(0) + ", end : " + cur.get(1));
		}

	}

}