LargestRectangleinHistogram

Change-Id: I2bf540dbfa158c9292cb7fdbe4f3c356d36fb1c7

Author: applewjg

Diff for: LargestRectangleinHistogram.java

@@ -0,0 +1,52 @@
+/*
+ Author:     Andy, [email protected]
+ Date:       Jan 6, 2015
+ Problem:    Largest Rectangle in Histogram
+ Difficulty: Hard
+ Source:     https://oj.leetcode.com/problems/largest-rectangle-in-histogram/
+ Notes:
+ Given n non-negative integers representing the histogram's bar height where the width of each 
+ bar is 1, find the area of largest rectangle in the histogram.
+ For example,
+ Given height = [2,1,5,6,2,3],
+ return 10.
+
+ Solution: 1. Only calucate area when reaching local maximum value.
+           2. Keep a non-descending stack. O(n). if the vector height is not allowed to be changed.
+*/
+public class Solution {
+    public int largestRectangleArea_1(int[] height) {
+        int res = 0;
+        for (int i = 0; i < height.length; ++i) {
+            if ((i < height.length - 1) && (height[i] <= height[i+1])) {
+                continue;
+            }
+            int minheight = height[i];
+            for (int j = i; j >= 0; --j) {
+                minheight = Math.min(minheight, height[j]);
+                res = Math.max(res, (i-j+1)*minheight);
+            }
+        }
+        return res;
+    }
+    public int largestRectangleArea(int[] height) {
+        int res = 0;
+        Stack<Integer> stk = new Stack<Integer>();
+        int i = 0;
+        while (i < height.length) {
+            if (stk.isEmpty() == true || (height[i] >= height[stk.peek()])) {
+                stk.push(i++);
+            } else {
+                int idx = stk.pop();
+                int width = stk.isEmpty() ? i : (i - stk.peek() - 1);
+                res = Math.max(res, width*height[idx]);
+            }
+        }
+        while (stk.isEmpty() == false) {
+            int idx = stk.pop();
+            int width = stk.isEmpty() ? height.length : (height.length - stk.peek() - 1);
+            res = Math.max(res, width*height[idx]);
+        }
+        return res;
+    }
+}