随手小代码——最大子数组 分治法

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本文链接：http://www.cnblogs.com/wlsandwho/p/4710986.html

耻辱墙：http://www.cnblogs.com/wlsandwho/p/4206472.html

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多年来总算自己亲手写了一次。

算法导论真是本好书。

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一直以为跨mid的部分会很难办，其实仔细看书亲手写过代码之后就会发现，也没啥。

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每个函数最后两个引用参数，是为了把最大的边界序号传回调用点。毕竟书上的是伪代码。

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  1 #include "stdafx.h"
  2 #include <intsafe.h>
  3 #include <iostream>
  4 
  5 using namespace std;
  6 
  7 int FindMaxCrossingSubArraybyWLS(int nArray[],int nLeft,int nMid,int nRight,int& nMaxLeftIndex,int& nMaxRightIndex)
  8 {
  9     int nLeftSum=INT_MIN;
 10     int nRightSum=INT_MIN;
 11     int nSum=0;
 12 
 13     for (int i=nMid;i>=nLeft;--i)
 14     {
 15         nSum+=nArray[i];
 16 
 17         if (nSum>nLeftSum)
 18         {
 19             nLeftSum=nSum;
 20             nMaxLeftIndex=i;
 21         }
 22     }
 23 
 24     nSum=0;
 25     for (int i=nMid+1;i<=nRight;i++)
 26     {
 27         nSum+=nArray[i];
 28 
 29         if (nSum>nRightSum)
 30         {
 31             nRightSum=nSum;
 32             nMaxRightIndex=i;
 33         }
 34     }
 35 
 36     return nLeftSum+nRightSum;
 37 }
 38 
 39 int FindMaxSubArraybyWLS(int nArray[],int nLeft,int nRight,int& nMaxLeftIndex,int& nMaxRightIndex)
 40 {
 41     int nMaxSum=INT_MIN;
 42 
 43     if (nLeft==nRight)
 44     {
 45         nMaxLeftIndex=nLeft;
 46         nMaxRightIndex=nRight;
 47 
 48         nMaxSum=nArray[nLeft];
 49     }
 50     else
 51     {
 52         int nMid=(nLeft+nRight)/2;
 53 
 54         int nLefMaxLeftIndex=0;
 55         int nLeftMaxRightIndex=0;
 56         int nLeftMaxSubArraySum=FindMaxSubArraybyWLS(nArray,nLeft,nMid,nLefMaxLeftIndex,nLeftMaxRightIndex);
 57 
 58         int nRightMaxLeftIndex=0;
 59         int nRightMaxRightIndex=0;
 60         int nRightMaxSubArraySum=FindMaxSubArraybyWLS(nArray,nMid+1,nRight,nRightMaxLeftIndex,nRightMaxRightIndex);
 61 
 62         int nMidMaxLeftIndex=0;
 63         int nMidMaxRightIndex=0;
 64         int nMidMaxSubArraySum=FindMaxCrossingSubArraybyWLS(nArray,nLeft,nMid,nRight,nMidMaxLeftIndex,nMidMaxRightIndex);
 65 
 66         if (nLeftMaxSubArraySum>nMidMaxSubArraySum && nLeftMaxSubArraySum>nRightMaxSubArraySum)
 67         {
 68             nMaxSum=nLeftMaxSubArraySum;
 69 
 70             nMaxLeftIndex=nLefMaxLeftIndex;
 71             nMaxRightIndex=nLeftMaxRightIndex;
 72         }
 73         else
 74         if (nRightMaxSubArraySum>nLeftMaxSubArraySum && nRightMaxSubArraySum>nMidMaxSubArraySum)
 75         {
 76             nMaxSum=nRightMaxSubArraySum;
 77 
 78             nMaxLeftIndex=nRightMaxLeftIndex;
 79             nMaxRightIndex=nRightMaxRightIndex;
 80         }
 81         else
 82         {
 83             nMaxSum=nMidMaxSubArraySum;
 84 
 85             nMaxLeftIndex=nMidMaxLeftIndex;
 86             nMaxRightIndex=nMidMaxRightIndex;
 87         }
 88     }
 89 
 90     return nMaxSum;
 91 }
 92 
 93 int _tmain(int argc, _TCHAR* argv[])
 94 {
 95 //　int nArr[7]={1,2,3,-1,4,-2,-3};
 96 //
 97 //　int nMaxLeftIndex=0;
 98 //　int nMaxRightIndex=0;
 99 //　int nMaxSum=FindMaxCrossingSubArraybyWLS(nArr,0,3,6,nMaxLeftIndex,nMaxRightIndex);
100 
101     int nArr[16]={13,-3,-25,20,-3,-16,-23,18,20,-7,12,-5,-22,15,-4,7};
102 
103     int nMaxLeftIndex=0;
104     int nMaxRightIndex=0;
105     int nMaxSum=FindMaxSubArraybyWLS(nArr,0,15,nMaxLeftIndex,nMaxRightIndex);
106 
107     cout<<"nArr["<<nMaxLeftIndex<<"..."<<nMaxRightIndex<<"] = "<<nMaxSum<<endl;
108 
109     return 0;
110 }

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一直反对abcijklmnpq的命名，所以我一般只在循环里用i。

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 千里黄云白日曛,北风吹雁雪纷纷。