SPOJ - GSS1, Segment Tree

2017/gss1.cpp

+#include <cstdio>
+#include <algorithm>
+ 
+#define MAXN 50050
+ 
+using namespace std;
+ 
+struct node {
+        int ans, mpref, msuf, sum;
+};
+ 
+node segtree[ 4*MAXN + 1 ];
+ 
+int N, A[ MAXN + 1 ];
+ 
+void init( int n, int l, int r ) {
+    if( l == r ) {
+        segtree[ n ].sum = A[ l ];
+        segtree[ n ].mpref = A[ l ];
+        segtree[ n ].msuf = A[ l ];
+        segtree[ n ].ans = A[ l ];
+    } else {
+        int mid = ( l + r ) >> 1;
+        init( 2*n, l, mid );
+        init( 2*n + 1, mid + 1, r );
+        segtree[ n ].sum = segtree[ 2*n ].sum + segtree[ 2*n + 1 ].sum;
+        segtree[ n ].ans = max( 
+        max( segtree[ 2*n ].ans, segtree[ 2*n + 1 ].ans ),
+        segtree[ 2*n ].msuf + segtree[ 2*n + 1 ].mpref );
+        
+        segtree[ n ].mpref = max( segtree[ 2*n ].mpref, 
+        segtree[ 2*n ].sum + segtree[ 2*n + 1 ].mpref );
+        
+        segtree[ n ].msuf = max( segtree[ 2*n + 1 ].msuf,
+        segtree[ 2*n + 1 ].sum + segtree[ 2*n ].msuf );
+    }
+}
+ 
+node query( int n, int l, int r, int i, int j ) {
+    if( l == i && r == j ) return segtree[ n ];
+    int mid = ( l + r ) >> 1;
+    if( j <= mid ) return query( 2*n, l, mid, i, j );
+    else if( i > mid ) return query( 2*n + 1, mid + 1, r, i, j );
+    node A = query( 2*n, l, mid, i, mid );
+    node B = query( 2*n + 1, mid + 1, r, mid + 1, j );
+    node C;
+    C.sum = A.sum + B.sum;
+    C.ans = max( max( A.ans, B.ans ), A.msuf + B.mpref );
+    C.mpref = max( A.mpref, A.sum + B.mpref );
+    C.msuf = max( B.msuf, B.sum + A.msuf );
+    return C;
+ 
+}
+ 
+int main( void ) {	
+    scanf("%d", &N );
+    for( int i = 1; i <= N; i++ ) {
+        scanf("%d", &A[ i ] );
+    }
+    init( 1, 1, N );
+    int Q;
+    scanf("%d", &Q );
+    while( Q-- ) {
+            int x, y;
+            scanf("%d %d", &x, &y );
+            node A = query( 1, 1, N, x, y );
+            printf("%d\n", A.ans );
+    }
+    return 0;
+}
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