0099 经典算法系列——线段树

 

LeetCode 307

class NumArray {
    int[] sum;
    int size;
    int[] nums;
    public NumArray(int[] nums) {
        if (nums == null || nums.length == 0) 
            return;
        this.nums = nums;
        this.size = nums.length;
        
        sum = new int[4*size];
        buildTree(1, 0, size - 1);
    }
    private void buildTree(int nodeId, int left, int right) {
        if (left == right) {
            sum[nodeId] = nums[left]; // += 改成 =
            return;
        }
        int mid = (left + right) >> 1;
        
        buildTree(nodeId << 1 , left, mid);
        buildTree(nodeId << 1|1 , mid + 1, right);
        sum[nodeId] = sum[nodeId << 1] + sum[nodeId << 1|1];
    }
    
    public void update(int i, int val) {
        updateSingle(1, val, i, 0, size - 1);
    }
    private void updateSingle(int nodeId, int val, int leftRight, int left, int right) {
        if (left == right) {
            nums[left] = val;
            sum[nodeId] = val;
            return;
        }
        int mid = (left + right) >> 1;
        if (leftRight <= mid && leftRight >= left)
            updateSingle(nodeId << 1, val, leftRight, left, mid);
        else
            updateSingle(nodeId << 1 | 1, val, leftRight, mid + 1, right); 
        
         sum[nodeId] = sum[nodeId << 1] + sum[nodeId << 1|1];
    }
    
    public int sumRange(int i, int j) {
        return query(1, i, j, 0, size - 1);
    }
    
    private int query(int nodeId, int qLeft, int qRight, int left, int right) {
        if (qLeft == qRight) return nums[qLeft];
        if (qRight < left || qLeft > right) return 0; //超出范围！
        if (qLeft <= left && qRight >= right){
            return sum[nodeId];
        }
        int mid = (left + right) >> 1;
        int lVal = query(nodeId << 1, qLeft, qRight, left, mid);
        int rVal = query(nodeId << 1|1, qLeft, qRight, mid + 1, right);
        return lVal + rVal;
    }
}

/**
 * Your NumArray object will be instantiated and called as such:
 * NumArray obj = new NumArray(nums);
 * obj.update(i,val);
 * int param_2 = obj.sumRange(i,j);
 */