LintCode 接雨水题目分析方法三

public class Solution {
    /**
     * @param heights: an array of integers
     * @return: a integer
     */
    public int trapRainWater(int[] A) {
        if(A == null || A.length < 3)
            return 0;
        
        int localMax = A[0];
        int[] left = new int[A.length];
        int[] right = new int[A.length];
        
        for(int i=0;i<A.length;i++) {
            if(A[i] <= localMax)
                left[i] = localMax;
            else {
                localMax = A[i];
                left[i] = localMax;
            }
        }
        
        localMax = A[A.length-1];
        for(int i=A.length-1;i>-1;i--) {
            if(A[i] <= localMax)
                right[i] = localMax;
            else {
                localMax = A[i];
                right[i] = localMax;
            }
        }
        
        int area = 0;
        for(int i=0;i<A.length;i++) {
            area += Math.min(left[i], right[i]) - A[i];
        }
        
        return area;
    }
}

public class Solution {
    /**
     * @param heights: an array of integers
     * @return: a integer
     */
    public int trapRainWater(int[] heights) {
        if(heights.length <= 2) return 0;
        int max = -1, maxInd = 0;
        int i = 0;
        for(; i < heights.length; ++i){
            if(heights[i] > max){
                max = heights[i];
                maxInd = i;
            }
        }
        int area = 0, root = heights[0];
        for(i = 0; i < maxInd; ++i){
            if(root < heights[i]) root = heights[i];
            else area += (root - heights[i]);
        }
        for(i = heights.length-1, root = heights[heights.length-1]; i > maxInd; --i){
            if(root < heights[i]) root = heights[i];
            else area += (root - heights[i]);
        }
        return area;
    }
}

public class Solution {
    /**
     * @param heights: an array of integers
     * @return: a integer
     */
    public int trapRainWater(int[] A) {
        if(A == null || A.length < 3)
            return 0;
        
        //两根指针一头一尾向中间进发
        int left = 0;
        int right = A.length-1;
        //两个变量存储左右两边的局部最大高度
        int leftMax = 0;
        int rightMax = 0;
        
        int area = 0;
        
        while(left <= right) {
            leftMax = Math.max(leftMax, A[left]);
            rightMax = Math.max(rightMax, A[right]);
            
            //小的那边可以存水
            if(leftMax <= rightMax) {
                if(leftMax > A[left])
                    area += leftMax - A[left];
                left++;
            }
            else {
                if(rightMax > A[right])
                    area += rightMax - A[right];
                right--;
            }
        }
        
        return area;
    }
}