leetcode53- Maximum Subarray- easy
阿新 • • 發佈:2017-11-11
負數 return ray lee ast 一個 num imu 轉變
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
.
1.前綴法。第一個優化是把連續和轉變為sum[j] - sum[i]的值。第二個優化是引入前綴,一直記錄前i個數裏最小的sum[i],那你每次算i~j和的時候肯定是要減去這個最小前綴才能讓差值最大化的。
2.貪婪法。每次如果發現當前的sum變成負數了就立即舍去,下一個數加進來的時候肯定選擇不要前面這串累贅的。
1.前綴法實現
class Solution { public int maxSubArray(int[] nums) { if (nums == null || nums.length == 0) { return 0; } int[] sums = new int[nums.length + 1]; sums[0] = 0; for (inti = 0; i < nums.length; i++) { sums[i + 1] = sums[i] + nums[i]; } int minPreSum = sums[0]; int maxSum = Integer.MIN_VALUE; for (int i = 1; i < sums.length; i++) { int crtSum = sums[i] - minPreSum; maxSum = Math.max(maxSum, crtSum); minPreSum= Math.min(minPreSum, sums[i]); } return maxSum; } }
2.貪心法實現
class Solution { public int maxSubArray(int[] nums) { if (nums == null || nums.length == 0) { return 0; } int maxSum = Integer.MIN_VALUE; int localSum = 0; for (int i = 0; i < nums.length; i++) { localSum += nums[i]; maxSum = Math.max(maxSum, localSum); localSum = Math.max(localSum, 0); } return maxSum; } }
leetcode53- Maximum Subarray- easy