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var NumArray = function(nums) {
this.nums = nums;
this.st = [];
this.constructST();
};
//construct segment tree :: start
NumArray.prototype.constructSTUtil = function(segmentStart, segmentEnd, segmentTreeIndex){
var mid;
// 如果数组只包含一个元素
// 将其存储与线段树的当前节点并返回
if(segmentStart === segmentEnd){
this.st[segmentTreeIndex] = this.nums[segmentStart];
return this.st[segmentTreeIndex];
}
// 如果有不止一个元素,
// 则递归计算左右子树,并将两者之和存储与节点内,并返回
mid = this.getMid(segmentStart,segmentEnd);
this.st[segmentTreeIndex] = this.constructSTUtil(segmentStart , mid ,segmentTreeIndex*2+1) +
this.constructSTUtil(mid+1, segmentEnd ,segmentTreeIndex*2+2);
return this.st[segmentTreeIndex];
}
NumArray.prototype.constructST = function(){
var n = this.nums.length;
if(n === 0){
return false;
}
this.constructSTUtil( 0, n-1, 0);
}
//construct segment tree :: end
//update array and segment tree :: start
NumArray.prototype.update = function(index, value){
var n = this.nums.length,
diff = value - this.nums[index]
this.nums[index] = value;
this.updateUtil(0,n-1,index,diff,0)
}
NumArray.prototype.updateUtil = function(segmentStart , segmentEnd , index , diff ,segmentTreeIndex){
if(index < segmentStart || index > segmentEnd){
return;
}
// 如果输入下标在节点范围之内,
// 则更新节点及其孩子的值
this.st[segmentTreeIndex] += diff;
if (segmentStart != segmentEnd){
var mid = this.getMid(segmentStart, segmentEnd);
this.updateUtil(segmentStart, mid, index, diff, 2*segmentTreeIndex + 1);
this.updateUtil(mid+1, segmentEnd, index, diff, 2*segmentTreeIndex + 2);
}
}
//update array and segment tree :: end
//get sum :: start
NumArray.prototype.sumRange = function(start ,end){
var self = this,
n = this.nums.length;
var getSumUtil = function(segmentStart ,segmentEnd , start, end, segmentTreeIndex){
var mid;
// 如果当前节点存储的线段是区间的一部分,
// 返回当前线段的和
if(segmentStart >= start && segmentEnd <= end){
return self.st[segmentTreeIndex];
}
if(start > segmentEnd || end < segmentStart){
return 0;
}
mid = self.getMid(segmentStart,segmentEnd);
return getSumUtil(segmentStart ,mid ,start , end, 2*segmentTreeIndex+1) +
getSumUtil(mid+1 ,segmentEnd ,start , end, 2*segmentTreeIndex+2)
}
return getSumUtil(0,n-1,start,end,0);
}
NumArray.prototype.getMid = function(start , end){
return start + Math.floor((end - start)/2);
}