segment tree building method

Question

Hi i am learning segment tree. Buildilding the segment tree using the recursive methodis clear to me , i have implemented it like this:

void build( int n, int b, int e){
    if(b > e) return;
    else if (b == e) { tree[n] = arr[b]; return }
    build(n*2 , b , (b+e)/2 );
    build(n*2+1 , (b+e)/2+1 , e );
    tree[n] =  tree[n*2] + tree[n*2 + 1] ;
}

but i have seen a shoter implementation like this :

    void build() {  // build the tree
      for (int i = n - 1; i > 0; --i) t[i] = t[i<<1] + t[i<<1|1];
   }

i understand that t[i<<1] is same as  t[2*i] and t[i<<1|1] is same as t[2*i+1]

but how does that logic help i building the segment tree?? a simple example would be very helpful

can you show the complete original code of "short implementation"? I think build() function only builds the other node except the leave, so the leave has initated — malioboro, Feb 22 '16 at 17:07
@malioboro here is the link : https://ideone.com/0K8XPI and here is the explanation that i was looking up: http://codeforces.com/blog/entry/18051 — kshitij singh, Feb 23 '16 at 03:27

score 0 · Accepted Answer · answered Feb 23 '16 at 03:35

0

Actually there is a different "function" of build() in your code and their code. In your code, when you build the segment tree you also input the leaves value, but in their code, they input the leaves value in this statement:

for (int i = 0; i < n; ++i) scanf("%d", t + n + i);

and use build() to only fill the other node

answered Feb 23 '16 at 03:35

malioboro

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thanks a lot i understood it. But how does the above statement fills the leave nodes .I mean how does above scanf() works ? – kshitij singh Feb 23 '16 at 05:13
2

it use pointer concept, simply said `scanf("%d", t + n + i)` same with `scanf("%d", &t[n+i])` – malioboro Feb 23 '16 at 05:19

segment tree building method

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