[Lintcode]Interval Sum II区间求和 II

Given an integer array in the construct method, implement two methods query(start, end) and modify(index, value):

• For query(start, end), return the sum from index start to index end in the given array.
• For modify(index, value), modify the number in the given index to value

Example

Given array A = [1,2,7,8,5].

• query(0, 2), return 10.
• modify(0, 4), change A[0] from 1 to 4.
• query(0, 1), return 6.
• modify(2, 1), change A[2] from 7 to 1.
• query(2, 4), return 14.

分析：按题目提示，最好先做过线段树的构造，线段树的查询，线段树的修改三道题，因为这道题分别涉及线段树的构造/修改/查询。第一个一次通过的困难题。

public class Solution {    /* you may need to use some attributes here */    class SegmentTreeNode {      public int start, end;      long val;      public SegmentTreeNode left, right;      public SegmentTreeNode(int start, int end) {          this.start = start;          this.end = end;          this.left = this.right = null;      }    }        private SegmentTreeNode tree = null;    /**     * @param A: An integer array     */    public Solution(int[] A) {        if(A == null) return;        tree = build(A, 0, A.length - 1);    }        private SegmentTreeNode build(int[] A, int start, int end) {        if(start > end) return null;        SegmentTreeNode res = new SegmentTreeNode(start, end);        if(start == end) {            res.val = A[start];        } else {            SegmentTreeNode left = build(A, start, (start + end) / 2);            SegmentTreeNode right = build(A, (start + end) / 2 + 1, end);            res.left = left;            res.right = right;            res.val = left.val + right.val;        }        return res;    }        /**     * @param start, end: Indices     * @return: The sum from start to end     */    public long query(int start, int end) {        return query(tree, start, end);    }        private long query(SegmentTreeNode curr, int start, int end) {        if(curr.start == start && curr.end == end) return curr.val;        if(start > curr.end || end < curr.start) return 0;                        long left = query(curr.left, start, Math.min(end, curr.left.end));        long right = query(curr.right, Math.max(start, curr.right.start), end);        return left + right;    }        /**     * @param index, value: modify A[index] to value.     */    public void modify(int index, int value) {        modify(tree, index, value);    }        private long modify(SegmentTreeNode curr, int index, int value) {        if(index < curr.start || index > curr.end) return 0;        if(curr.start == curr.end && curr.start == index) {            long diff = value - curr.val;            curr.val = value;            return diff;        } else {            if((curr.start + curr.end) / 2 >= index) {                long diff = modify(curr.left, index, value);                curr.val = curr.val + diff;                return diff;            } else {                long diff = modify(curr.right, index, value);                curr.val = curr.val + diff;                return diff;            }        }    }}