public class MaxSubSeqSum {/*** 算法1，窮舉搜索*/public static final int maxSubSeqSum1(int seq[]) {int length = seq.length;int sum = 0;for (int i = 0; i < length; i++) {for (int j = i; j < length; j++) {int theSum = 0;for (int k = i; k <= j; k++) {theSum += seq[k];}if (sum < theSum) sum = theSum;}}return sum;}/*** 算法2。改進算法1*/public static final int maxSubSeqSum2(int seq[]) {int length = seq.length;int sum = 0;for (int i = 0; i < length; i++) {int theSum = 0;for (int j = i; j < length; j++) {theSum += seq[j];if (theSum > sum) {sum = theSum;}}}return sum;}/*** 算法3：分治法*/public static final int maxSubSeqSum3(int seq[]) {return maxSubSeqSum31(seq, 0, seq.length - 1);}private static int max(int... args) {int max = Integer.MIN_VALUE;for (int i : args) {if (i > max) max = i;}return max;}private static final int maxSubSeqSum31(int[] seq, int left, int right) {if (right - left == 1) { // 僅僅剩下兩個了：return max(seq[left], seq[right], seq[left] + seq[right], 0);} else if (left == right) {return max(seq[left], 0);} else {int middle = (left + right) / 2;int maxLeft = maxSubSeqSum31(seq, left, middle);int maxRight = maxSubSeqSum31(seq, middle + 1, right);int maxMiddle = maxSubSeqMiddle(seq, left, right, middle);return max(maxLeft, maxRight, maxMiddle);}}private static final int maxSubSeqMiddle(int[] seq, int left, int right, int middle) {int maxLeft = 0, maxRight = 0;int temp = 0;for (int i = middle; i >= left; i--) {temp += seq[i];if (maxLeft < temp) {maxLeft = temp;}}temp = 0;for (int i = middle + 1; i <= right; i++) {temp += seq[i];if (maxRight < temp) {maxRight = temp;}}return max(maxLeft, maxRight, maxLeft + maxRight);}/*** 最快的計算方式* @param seq* @return*/public static final int maxSubSeqSum4(int seq[]) {int sum = 0, theSum = 0;int length = seq.length;for (int i = 0; i < length; i++) {theSum = max(theSum + seq[i], 0);if (sum < theSum) {sum = theSum;}}return sum;}public static void main(String[] args) throws Exception {int length = 100000;int[] arr = new int[length];Random rand = new Random(10);for (int i = 0; i < length; i++) {arr[i] = rand.nextInt(11) - 5;}long t = System.currentTimeMillis(); // int ret1 = maxSubSeqSum1(arr); // System.out.println("算法1耗時：" + (System.currentTimeMillis() - t)); // System.out.println("結果1：" + ret1); // t = System.currentTimeMillis();int ret2 = maxSubSeqSum2(arr);System.out.println("算法2耗時：" + (System.currentTimeMillis() - t));System.out.println("結果2：" + ret2);t = System.currentTimeMillis();int ret3 = maxSubSeqSum3(arr);System.out.println("算法3耗時：" + (System.currentTimeMillis() - t));System.out.println("結果3：" + ret3);t = System.currentTimeMillis();int ret4 = maxSubSeqSum4(arr);System.out.println("算法4耗時：" + (System.currentTimeMillis() - t));System.out.println("結果4：" + ret4);} }

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