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struct segment_tree
{
    struct NODE{
        int st , EN, value;// start , end
 
         void init(int L,int R){
           st = L, EN = R; 
           if(L == R) value = a[L];
         }
    }g[3*N];
 
    void fill_CN(NODE &CN, NODE &LN, NODE &RN) // fill_current_node
    {
        CN.value = LN.value + RN.value;
    }
 
    void build(int CN,int L,int R)
    {
        g[CN].init(L,R);
 
        if(L ==R ) return;
        int mid = (L+R)>>1;
        int LN = CN << 1;
 
        build(LN,L,mid);
        build(LN|1,mid+1,R);
 
        fill_CN (g[CN], g[LN] , g[LN|1]);
    }
 
    void update(int CN, int pos,int val)
    {
        if(g[CN].st == g[CN].EN)
        {
            g[CN].value = val;
            return;
        }
 
        int mid = (g[CN].st + g[CN].EN)>>1;
 
        int LN = CN << 1 ;
        if(mid >= pos) update(LN, pos,val);
        else update(LN|1,pos,val);
 
        fill_CN(g[CN], g[LN],g[LN|1]);
    }
 
    int query(int CN, int L,int R)
    {
        int x = g[CN].st;
        int y = g[CN].EN;
        if(y < L || x > R) return 0;
 
        if(L <= x && R >= y ) return g[CN].value;
 
        int LN = CN<<1;
 
        int ans = query(LN,L,R);
        ans += query(LN|1,L,R);
        return ans;
    }
 
} s_tree;

prog.cpp:10:10: error: ‘N’ was not declared in this scope
     }g[3*N];
          ^
prog.cpp: In member function ‘void segment_tree::NODE::init(int, int)’:
prog.cpp:8:31: error: ‘a’ was not declared in this scope
            if(L == R) value = a[L];
                               ^
prog.cpp: In member function ‘void segment_tree::build(int, int, int)’:
prog.cpp:19:9: error: ‘g’ was not declared in this scope
         g[CN].init(L,R);
         ^
prog.cpp: In member function ‘void segment_tree::update(int, int, int)’:
prog.cpp:33:12: error: ‘g’ was not declared in this scope
         if(g[CN].st == g[CN].EN)
            ^
prog.cpp:39:20: error: ‘g’ was not declared in this scope
         int mid = (g[CN].st + g[CN].EN)>>1;
                    ^
prog.cpp: In member function ‘int segment_tree::query(int, int, int)’:
prog.cpp:50:17: error: ‘g’ was not declared in this scope
         int x = g[CN].st;
                 ^

Standard output is empty