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package class25;
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import java.util.Stack;
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public class Code02_AllTimesMinToMax {
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public static int max1(int[] arr) {
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int max = Integer.MIN_VALUE;
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for (int i = 0; i < arr.length; i++) {
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for (int j = i; j < arr.length; j++) {
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int minNum = Integer.MAX_VALUE;
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int sum = 0;
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for (int k = i; k <= j; k++) {
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sum += arr[k];
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minNum = Math.min(minNum, arr[k]);
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}
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max = Math.max(max, minNum * sum);
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}
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}
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return max;
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}
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public static int max2(int[] arr) {
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int size = arr.length;
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int[] sums = new int[size];
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sums[0] = arr[0];
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for (int i = 1; i < size; i++) {
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sums[i] = sums[i - 1] + arr[i];
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}
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int max = Integer.MIN_VALUE;
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Stack<Integer> stack = new Stack<Integer>();
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for (int i = 0; i < size; i++) {
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while (!stack.isEmpty() && arr[stack.peek()] >= arr[i]) {
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int j = stack.pop();
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max = Math.max(max, (stack.isEmpty() ? sums[i - 1] : (sums[i - 1] - sums[stack.peek()])) * arr[j]);
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}
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stack.push(i);
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}
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while (!stack.isEmpty()) {
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int j = stack.pop();
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max = Math.max(max, (stack.isEmpty() ? sums[size - 1] : (sums[size - 1] - sums[stack.peek()])) * arr[j]);
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}
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return max;
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}
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public static int[] gerenareRondomArray() {
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int[] arr = new int[(int) (Math.random() * 20) + 10];
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for (int i = 0; i < arr.length; i++) {
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arr[i] = (int) (Math.random() * 101);
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}
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return arr;
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}
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public static void main(String[] args) {
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int testTimes = 2000000;
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System.out.println("test begin");
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for (int i = 0; i < testTimes; i++) {
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int[] arr = gerenareRondomArray();
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if (max1(arr) != max2(arr)) {
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System.out.println("FUCK!");
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break;
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}
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}
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System.out.println("test finish");
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}
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// 本题可以在leetcode上找到原题
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// 测试链接 : https://leetcode.com/problems/maximum-subarray-min-product/
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// 注意测试题目数量大，要取模，但是思路和课上讲的是完全一样的
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// 注意溢出的处理即可，也就是用long类型来表示累加和
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// 还有优化就是，你可以用自己手写的数组栈，来替代系统实现的栈，也会快很多
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public static int maxSumMinProduct(int[] arr) {
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int size = arr.length;
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long[] sums = new long[size];
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sums[0] = arr[0];
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for (int i = 1; i < size; i++) {
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sums[i] = sums[i - 1] + arr[i];
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}
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long max = Long.MIN_VALUE;
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int[] stack = new int[size];
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int stackSize = 0;
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for (int i = 0; i < size; i++) {
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while (stackSize != 0 && arr[stack[stackSize - 1]] >= arr[i]) {
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int j = stack[--stackSize];
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max = Math.max(max,
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(stackSize == 0 ? sums[i - 1] : (sums[i - 1] - sums[stack[stackSize - 1]])) * arr[j]);
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}
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stack[stackSize++] = i;
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}
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while (stackSize != 0) {
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int j = stack[--stackSize];
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max = Math.max(max,
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(stackSize == 0 ? sums[size - 1] : (sums[size - 1] - sums[stack[stackSize - 1]])) * arr[j]);
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}
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return (int) (max % 1000000007);
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}
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}
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